For all its heartache and heroism, the remarkable film capturing the last days of 21-year-old Alex Lewis might have remained unseen, outside of his family.

Oxfordshire documentary maker David Dugan had known Alex and his family – who like him had settled in the village of Brightwell-cum-Sotwell – for more than two decades.

Alex had been diagnosed with bone cancer shortly before his 18th birthday, when still a sixth-former at Wallingford School. And as his three-year battle with the disease drew to its tragic and inevitable end, Mr Dugan had offered to make a film for the Lewis family.

“The whole village knew and loved Alex. Over the three years he had undergone intensive treatment and last summer it reached the stage where he was running out of options,” recalled Mr Dugan.

“You feel so helpless. I asked if there was anything I could do. And I realised the only way I could help was to ask the family if they would like me to make a film.”

His idea was to provide the Lewis family with precious memories, recording Alex talking and capturing his continuing zest for life and “that smile” which spread across his face, even in the darkest of times.

Alex readily embraced the idea, hoping that filming his story would one day assist the campaign to raise money for research into bone cancer.

But then Alex met Ali, and the film-maker knew he was witnessing a remarkable romance, a love affair with the power to move and inspire a national television audience.

The Lewis family too recognised that the film Mr Dugan was making deserved to be seen by a much larger audience as a tribute to their son and his young bride — and Alex: A Life Fast Forward will be shown on Thursday, July 21, at 9pm on BBC3.

Mr Dugan, whose science and history documentaries have won a string of awards, including three Emmys, began filming in June 2010.

Alex’s approach to his illness was to cram as much as possible into the limited time that he had left, so Mr Dugan was not surprised to arrive and find the young man busily preparing to drive off to a party in Swansea.

What Mr Dugan could not have guessed was that his terminally-ill friend, who he had watched grow up, would kiss a girl that night and fall in love in the most emotionally-charged circumstances imaginable.

After that party kiss Alex and Alison Strain became inseparable.

By September they were engaged and they were to marry on Alex’s 22nd birthday in February.

As it was to turn out, Alex died just five days after the wedding, which was held in the village church.

The story of their love affair is at the heart of a film that shows us how a young man confronted his own mortality and the response of his loving family and friends.

“It was just an amazing coincidence that they met on the day that we started filming,” said Mr Dugan, chairman of Windfall Films. “We filmed him going off to Swansea in the car. He had met Ali when they had both been in Australia and he saw her again at that party.

“She really gave him a new lease of life. It was incredible to witness their relationship unfold. But there was a tragic aspect. Up until then Alex had been living each day as it came. Now he suddenly had a future to look forward to.

“But we never see him feeling sorry for himself. He was not only courageous but incredibly thoughtful towards all those around him.”

Mr Dugan’s recent productions include Inside Nature’s Giants and The Operation: Surgery Live, both for Channel 4. He also developed what some claim to be the first reality TV show, The Tourist Trap.

This radical format used hidden cameras to challenge national stereotypes, by comparing the behaviour of four nations on holiday.

But his personal involvement with the family meant he took an entirely different approach with this project.

“I was very careful not to infringe on anything that the family did not want. Because Alex knew me so well, he trusted me. I showed the family the film to make sure they were happy with it. I think they are.”

Alex’s parents, Andy and Angela Lewis, admit that it had been a tough decision to allow the film to be made and then go out on national television.

Mrs Lewis said: “We are very private people and it was difficult to share our situation. When I saw documentaries on families, I would say ‘why do people do this?’ But Alex wanted to do it.

“We feel the film has really done our son justice. A major reason behind Alex’s participation in the film was to raise awareness of primary bone cancer and the excellent work of the Bone Cancer Research Trust.

“Around 450 people are diagnosed with bone cancer each year in the UK and Ireland, mainly affecting children and young people between the ages of ten and 24. A recent publication has shown that there has been no improvement in survival for people with bone cancer in over 20 years.”

Alex, also a former St Birinus School pupil, was diagnosed only after complaining of pains in his arms over months. A keen sportsman and talented tennis player, he had put it down to a simple muscle injury before a physiotherapist suspected the real cause.

Mrs Lewis is full of praise for her daughter-in-law, who now works for Mr Dugan’s company. “It was incredible that Alex was to have this wonderful relationship, something he did not think possible. As the relationship progressed Ali came to live with us. It was an amazing thing for Ali’s family to really get behind her.”

Alex’s sudden deterioration meant that the wedding had to be brought forward. By then his handsome face is puffy and he struggles with his breathing at the reception afterwards. He watches his friends dance and joins in the singing to a Fleetwood Mac song.

But there are no tears. I suspect there will be plenty in the nation’s living rooms when Mr Dugan’s film goes out.