Taking on a challenge is irresistible to Alex Jones, whether it’s wrestling with a tricky romantic situation or risking life and limb hanging off the side of a mountain. She reveals to Jaine Blackman how her ‘ditzy’ on-screen image hides a steely core

Alex Jones is trying to stay calm as she talks about dangling from a rope, thousands of feet up a mountain.

But unsurprisingly, as the petite presenter of BBC One’s The One Show prepares to climb the 1,200ft, virtually sheer sandstone face of Moonlight Buttress, Utah – an ascent many seasoned professional climbers would baulk at – a few nerves do seep through.

The climb, to be shown on BBC One on March 17-20, will raise money for Sport Relief, the fundraising event running on alternate years with Comic Relief. Since Sport Relief began in 2002 many celebrities have pushed themselves to the limit for this cause – Davina McCall recently completed a gruelling seven-day, 500-mile endurance test. But Jones’s project can’t help but feel that bit more perilous.

Her family and boyfriend are openly nervous about the risk she’s running in the merciless conditions and climates of the mountains, and even 36-year-old Jones admits, in her lilting Welsh accent, that she’s “petrified”.

“I’ll be in a tiny tent at night which will be just hanging and literally tied to a ledge on the rock face – even though if it falls off the ropes will hold it, I’ll still end up dangling in space. So sleep I imagine will be impossible. And I have no past experience of climbing – I may have climbed a tree as a kid but that’s about it. I hated sport at school because I feel the cold and used to sneak off and sit on a radiator. I certainly didn’t turn into a gym bunny as an adult either.”

Even so, it was still an unwelcome shock for Jones when – despite a previous charity challenge last winter, a punishing 700-mile rickshaw ride around the UK for Children In Need – experts said she was 16 per cent below the average fitness level for a woman her age.

“It was a real wake-up call,” she says.

Undeterred though, Jones committed to intensive training, and now – hopefully anyway – she feels ready to take on her challenge.

“I hope I’ve done enough because no one will actually be beside me as I climb.

“The rest of the team will be making their way up above me, and it’ll be just down to me to try to remember all the holds and to make sure myself that the ropes that hold me are secure.

“Gripping the rock face and tying knots and grappling with ropes will be particularly hard for me as I suffer from Raynaud’s, a condition which means in cold conditions I suffer terribly from pain in my hands as my blood flow constricts.

“On the rock at night it will be seven degrees below.”

So far, so worrying.

And that’s before you also realise that, despite working out in an indoor climbing centre since January, Jones will only have undertaken one outdoor climb, in Pembrokeshire, prior to her three-day ordeal in America.

You suspect Jones also welcomes the opportunity to reveal there’s more grit to her than her warm, bubbly on-screen persona might suggest.

“I probably sometimes come across as quite ditzy,” she admits, “but underneath I can be quite determined and very focused.”

“As far as I’m concerned, it’s not ditziness anyway, rather it’s just a lightness of touch. I know people may say, ‘Oh, that Alex, she’s an airhead’, but I think to myself, ‘Ok, you come and do my job and see how you get on with it!’.”

Certainly, her success on The One Show, which she co-hosts with Matt Baker and with Chris Evans on Fridays, demonstrates that she’s a talented and skilled presenter.

It was no small feat for Carmarthenshire-born Jones, whose previous TV experience was in Welsh language children’s shows, to step into the shoes of Christine Bleakley, who left in 2010.

“It’s taken time, but in the last year I’ve finally really felt completely comfortable and at ease in the role.

She puts this progression down to being natural and not acquiring an artificial image.

“What you see is what you get with me – I’m no different off-screen. I like to be as natural as possible. Okay, sometimes I say the most ridiculous things, which Matt and Chris tease me about, but that’s just me.

“I’m conscientious and work my socks off to make sure I improve all the time. I’m well aware I have a job millions of people envy, and I’m determined to show I deserve it.”

Her love-life is equally envy-inducing. Jones clearly adores her boyfriend of two years, insurance broker Charlie Thomson, who’ll be at the top of the rock in Utah to greet her when she finishes.

“He’s so lovely and very proud of me for taking on the climb. He keeps telling me, ‘You’re the most determined person I’ve ever met, I know you can do this’. I’m sure I won’t be able to wait to fall into his arms.”

Jones admits their relationship is “complicated”, because Thomson is from New Zealand and would one day like to return. But while Jones is aware of the issue she isn’t prone to dwell on negatives.

She’s philosophical too about her “biological clock ticking” – she will turn 37 while she does her American climb.

“A family when the time is right would be amazing, and if it happened we’ll deal with that. But at the moment it doesn’t feel the right time to step away from my career.”

“I know I’ve got an awful lot to be grateful for. For now though, I just want to get down from that mountain safely!”

Sponsor Alex Jones at www.sportrelief.com/alex. Her climb will be shown on BBC One’s The One Show from March 17, 7pm.