Comedian Bill Maher claims to be looking for answers in Religulous (2008). But, as he tours the world compelling people to justify their faith, it quickly becomes clear that he reached his conclusions long before he began posing the questions and, thus, while it may land a few blows on organised religion, this self-satisfied mockumentary is as entrenched in its beliefs as its satirically soft targets.

Raised by a Jewish mother and Catholic father, Maher went to weekly mass until he was 13, since when his scepticism has petrified into contempt. In his defence, he's an equal opportunities decrier, as Judaism, Islam, Mormonism, Scientology and a range of ancient cults are ridiculed with the same hostility as Christianity. But Maher seems more intent on deriding the believers he interviews than confronting them with the flaws inherent in their faiths. Consequently, he appends sniping captions to talking-head sequences and cuts away to clips from overwrought devotional movies to lampoon the opinions being expressed.

Some of the juxtapositions are amusing, but this is an underhand tactic and the laughter is often infelicitous - as it was in the Sasha Baron Cohen vehicle, Borat: Cultural Learnings of America for Make Benefit Glorious Nation of Kazakhstan, which was also directed by Larry Charles (who exposed hypocrisy and the ludicrousness of human pomposity to much smarter effect in Larry David's lacerating sitcom, Curb Your Enthusiasm). There are moments of Boratian stunt comedy, such as Maher's parody of a proselytising Scientologist at Speaker's Corner in London. But the most striking pronouncements come without Maher's undue provocation, among them John Westcott's views on homosexuality, evangelical senator Mary Pryor's insight into why faith is good and the admission of a Vatican priest that Jesus only came sixth in a poll on who Italians would pray to in an emergency.

The footage of the re-enactment of the Passion at the Holy Land Experience theme park in Orlando, Florida is also arresting. But Maher never causes jaws to drop as Heidi Ewing and Rachel Grady did with Jesus Camp. Moreover, he spares Eastern belief systems his wrath. Erring on the Morgan Spurlock side of Michael Moore, he seems content to play on liberal prejudice and elicit easy yucks rather than assemble a coherent case against one faith in particular or religion in general. Consequently, he succeeds solely in reinforcing secular preconception and making some unsuspectingly sincere people look exceedingly foolish.

Franny Armstrong also finds herself preaching to the converted in The Age of Stupid (2009), a well-meaning, but hectoringly fussy attempt to urge the world to avert environmental Armageddon.

In 2030, last man standing Pete Postlethwaite was installed in The Global Archive, a storage facility designed to preserve humanity's finest achievements from the ravages of climate change. It's now 2055 and, surrounded by artworks, movies, specimen species and scientific data and discoveries, Postlethwaite sits before a giant video bank to record a message for anyone or anything that chances upon Earth after his demise. Summoning clips from a drag-and-display screen, he laments the ignorance and arrogance of post-millennial humanity, who ignored the portents and warnings and continued to accelerate global warming, deplete natural resources and consume itself out of existence.

Although the CGI images of the archival tower look homemade and the periodic animated inserts are as gimmicky as they're illuminating, this innovative picture is often instructive. Filmed in six countries over three years, the footage has been neatly shaped to seem like the work of divers hands. But the message is consistent throughout and Postlethwaite strives to avoid lecturing by adopting a tone of weary fury at the thoughtless destruction of the planet.

But what makes this such a depressing experience is that it is far from being the first documentary to try and prick the collective conscience of the political, commercial and industrial communities in the hope that they will collaborate on initiatives to halt what many experts fear will otherwise be the inevitable. Armstrong's doomsday gambit is bold and newsworthy. But it will take more than some futuristic finger wagging to fire the zeal of anyone powerful enough to make a difference in the time remaining to us.

Anne Feinsilber also adopts a novel approach to actuality in Requiem for Billy the Kid (2006), as she conducts a conversation with Henry McCarty (alias William Bonney) in seeking to ascertain whether Pat Garrett really did gun down his onetime friend at Fort Sumner or whether he escaped to Mexico before eking out his days as either Brushy Bill Roberts in Hico, Texas or John Miller in Prescott, Arizona. Not content with attempting to solve one of the Wild West's most enduring mysteries, Feinsilber also strives to forge a spiritual link between Billy and Arthur Rimbaud, the recklessly iconoclastic poet who also died at 21 with his potential scarcely fulfilled.

Unsurprisingly, she fails on both counts. Rimbaud's verses provide a poignant commentary on the Kid's troubled existence, but the comparison is as strained as the evidence that Billy reached a ripe old age is tenuous. Feinsilber is on much surer ground, however, in chronicling McCarty's odyssey from New York to Lincoln County and his involvement in the 1877 turf war that erupted when English cattle baron John Tunstall was gunned down by four acolytes of merchants Lawrence Murphy and James Dolan. She also offers some shrewd insights into the fateful events of 14 July 1881, thanks to the contributions of lawmen Tom Sullivan, Steven Sederwall and Gary Graves and the descendants of the key players in Billy's legend. But she is equally intrigued by the outlaw's movie mythologisation in the likes of Arthur Penn's The Left Handed Gun (1958) and Sam Peckinpah's Pat Garrett & Billy the Kid (1973), and she's well served by the recollections of the latter's screenwriter and star, Rudy Wurlitzer and Kris Kristofferson.

With Patrick Ghiringhelli's New Mexican vistas a consistent treat and Claire Diterzi's evocative score topped off by a husky rendition of Bob Dylan's `Knocking on Heaven's Door', this makes for fascinating viewing. It may not always be persuasive, but there's something sobering about the contention that Garrett's triumph set America on the road to the uncompromising conservatism that swept Reagan and Bush to office.

Feinsilber clearly shows promise, but she has a way to go to merit comparison with France's leading documentarists, Agnès Varda, Chris Marker, Raymond Depardon and Nicolas Philibert. The latter's best-known film, Etre et Avoir (2002), is a charming study of compassion and professionalism that retains its fascination in spite of the subsequent controversy that engulfed it. In spring 2000, Philibert was planning a documentary on the impact of European agricultural policy on impoverished French farmers. But during his research, he became increasingly taken with the single-class schools that continued to serve around 400 rural communities. In particular, he was intrigued by how a teacher could simultaneously sustain the education of a dozen or so pupils of varying ages, backgrounds and abilities, while also attending to their emotional needs at such differing levels of development.

Having visited nearly 100 schools, he opted for the one in Puy-du-Dome, in the Auvergne region of the volcanic Massif Central. Comprising 13 children, from nursery age to 11 year-olds, it was run by Georges Lopez, who was approaching retirement, having spent 20 years in the village, and whose rather traditional methods had earned him the respect of his neighbours. Shooting over ten weeks, Philibert accumulated almost 60 hours of footage and what emerged was a fascinating portrait of a consummate educator, who still took pleasure in seeing students learn and whose ability to reach each child on their own level was repaid with an affection that is readily evident.

Using film rather than digi-video, Philibert reclaims actuality from the sensation seeking of reality TV, as he captures the changing seasons to emphasise the unwavering support that Lopez gives to the moppets in his charge. Admonition is gentle and fair, reassurance is unceasing and the emotion Lopez feels at the end of term is wholly genuine (despite accusations that some of the scenes were staged for the benefit of the camera). But Philibert is equally interested in how the kids respond to Lopez and his admirably diverse curriculum and is rewarded with such memorable sequences as the day trip to the nearby `big' school and the arrival of next year's timorous intake.

Philibert's great gift is putting people at their ease so that they behave naturally before his lens and one young British film-maker has clearly learned this lesson, if Three Films by Marc Isaacs is anything to go by. Having served his apprenticeship with former Brookes Creative Arts Fellow, Pawel Pawlikowski, Isaacs debuted with Lift (2001), in which he strikes up conversations with the residents of an East End high-rise, as they ride the elevator between floors. Although a handful of drunken revellers prove confrontational, the garrulous insouciance with which the majority accept the stranger's presence is as heartening as their readiness to confide intimate details and opinions.

If this curiously recalls Tony Hancock's earnest discourse about nothing in particular with Hugh Lloyd in the hospital waiting room in The Blood Donor, the tone becomes more personal in Travellers (2002), even though the setting is the markedly less intimate surroundings of the South Yorkshire railway network. Once again Isaacs gains the confidence of his interlocutors with finessed ease. Yet while they reveal their views on true love with remarkable alacrity and honesty, there's never a suggestion that their responses are in any way motivated by a desire to seize their 15 minutes of fame.

Unfortunately, a less attractive side of the English character emerges in Calais: The Last Border (2003), as the booze cruisers to the French port and the expats trying to make a living there offer their jaundiced ten pennyworth on the migrants from the closed Sangatte camp desperately trying to cross the Channel. Isaacs is clearly drawn to the Iraqi whose entire family perished in a rocket attack on Kabul and the Jamaican whose visa has been invalidated by bureaucratic tinkering. But he is too aware that everyone's life is something of a struggle to pass any damning judgements.

Isaacs fits neatly into the tradition of British documentary making that was established in the 1930s and echoes of his approach reverberate through the last two collections in the BFI's tribute to the GPO Film Unit, We Live in Two Worlds and If War Should Come.

Covering the period 1936-38, the first two-disc set includes William Coldstream's enchanting musical, The Fairy of the Phone (1936), Alberto Cavalcanti's JB Priestley soapbox, We Live in Two Worlds (1937), Pat Jackson's The Horsey Mail and the incomparable Humphrey Jennings's Penny Journey - The Story of a Post Card from Manchester to Graffham (both 1938). Also on offer is a glorious selection of animations by the ever-inventive Norman McLaren (Love on the Wing, Book Bargain, News for the Navy, Mony a Pickle), experimentalist extraordinaire Len Lye (Rainbow Dance, Trade Tattoo, N or NW) and the peerless silhouette stylist, Lotte Reiniger (The HPO and The Tocher). But the highlights have to be the collaborations between composer Benjamin Britten and poet WH Auden: Harry Watt and Basil Wright's enduringly thrilling Night Mail (1936) and the little-seen 1938 offering, God's Chillun, which uses footage by Wright to illustrate an account of the slave trade and the development of the Caribbean in the century after emancipation.

In 1940, the GPO Film Unit was transferred to the Films Division of the Ministry of Information and renamed the Crown Film Unit. However, its last titles were among its most memorable, with Cavalcanti ruminating upon the laying of an underground cable between Amersham and Aylesbury in A Midsummer Day's Work, Jennings reflecting on hobbies and leisure in Spare Time and Jennings and Jackson bolstering the nation's Phoney War confidence in The First Days (all 1939). Jennings caught the mood of defiance again in Spring Offensive and London Can Take It! (both 1940), with the latter's indomitability recurring in co-director Harry Watt's Christmas Under Fire (1941).

Another notable inclusion is David MacDonald's Men of the Lightship (1940), which was one of the MOI films that Alfred Hitchcock helped tailor for the US market, after it had been rejected by both RKO and 20th Century-Fox. Roping in Rebecca scenarist Robert Sherwood and Mr & Mrs Smith star Robert Montgomery, Hitch spent $4,428 of his own money revising this tribute to the heroic crew of the East Dudgeon lightship. But he refused reimbursement and Men of Lightship 61 was released without his name among the credits.

The 70th anniversary of the Nazi invasion of Poland on 1 September 1939 is bound to be accompanied by a wealth of commemorabilia and not all of it will be as tasteful or worthwhile as World War II: I Was There. The passing this year of Great War veterans Bill Stone, Henry Allingham and Harry Patch has provided a sad reminder of the value of oral history and the six films in this compelling series provide eye-witness accounts of Dunkirk, the Blitz, the Battle of Britain and D-Day, as well as recollections of life in the Home Guard and the inspirational leadership of Winston Churchill.

Perhaps the most astonishing episode recalled here is Operation Dynamo, when 338,226 men were ferried back to Blighty over nine days in 222 naval vessels and 665 civilian craft. But the exploits of KW Mackenzie, VR Snell and Ginger Lacy, as they soared over the Home Counties to repel Operation Sealion, are every bit as heroic, with the latter's description of how he downed the Heinkel He 111 that bombed Buckingham Palace on 13 September 1940 laudably avoiding undue triumphalism. Walter Higgs's reminiscences about diffusing unexploded bombs in London are similarly matter-of-fact. But the pride in the men who served beside him is readily evident, as is the case of the Chatham Home Guard, who are keen to dispel the Dad's Army myth.

Nothing will ever surpass The World at War when it comes to chronicling the battle against the Axis. But this prioritisation of the British perspective serves as a necessary corrective to US series like Ken Burns's The War, which seem to think that Uncle Sam pretty much saved the world on his own.