Who’d be David Walliams? Even to a mind filled with sly comic genius, nothing can look very funny this weekend as he waits for the telephone call that will summon him to the southernmost tip of Spain and thence on a gruelling, six-hour swim from Europe to Africa.

The tall, elegant half of Little Britain (Matt Lucas is the short, fat, bald one) is about to join James Cracknell, the double Olympic gold medallist, on what looks to be a personal challenge of quite lunatic proportions.

Cracknell, who set off from Dover last Wednesday, is attempting to reach Africa in seven days by rowing, cycling and swimming. He capsized in rough seas moments into the 22-mile crossing to Cap Griz Nez, but eventually made it to the other side and is now pedalling furiously through France and Spain, stopping for only four hours a day.

After cycling 1,400 miles he will struggle into a wet suit, ready to swim the final leg.

Which is where Walliams comes in. Having swum the English Channel for Comic Relief in 2006 – raising more than £1m – it is his job, swimming alongside, to act as friend, coach and cheerleader to get an exhausted Cracknell to the finish. He admits the responsibility is weighing heavy, but dismisses it with dry humour. “It’s really James’s challenge,” he says. “I’m just jumping in at the end to get some of the glory.”

The pair met for a couple of practice sea swims off the south coast of France 10 days ago – two hours in the morning, three in the afternoon – and found they had an instant rapport. Walliams, 36, told Cracknell he was impressed at him having two Olympic medals. Cracknell, 35, said that was nothing: he knew someone with five. Cracknell, more used to rowing, admired Walliams’s natural swimming style and his ability to complete sporting challenges without being super-competitive. Walliams told him he could have been really good at sport if he’d wanted to – he just wasn’t a show-off.

Joking apart, what they face as they plough their way across the Strait of Gibraltar won’t be a breeze for either. Walliams swam the Channel in summer, with the water at a bearable 15C, this time it will be 10C at best. “In other words, you’d be dead in an hour if you fell in and hung around,” he says.

The swim promises to be as much an emotional as a physical challenge for Walliams, whose father Peter died just before Christmas. When he swam the Channel, both his parents were there to wave him off and later said seeing him complete the swim was their proudest moment.

Then, last September, Peter, a former London Transport engineer, was diagnosed with an aggressive form of liver cancer. Walliams cancelled all his immediate plans in order to spend time with him and the writers’ strike fortuitously postponed a trip to America to film a Stateside version of Little Britain. Within weeks, Walliams’s father was dead.

“I was having nightmares about it last night – it has been on my mind a lot,” he says. “If I had one ambition it would be to make my parents proud because they’ve always been very much part of my achievements. It will be very sad that my dad won’t be here to see us off.”

Walliams, for all his strange, creepy alter egos in Little Britain – the warped young man who fancies his friend’s granny, transvestite Emily Howard (“I’m a laydee”) and Sebas-tian Love, the parliamentary aide with an alarming crush on the prime minister – and his teasing hints about his sexuality (he once said he was “70% straight”) seems touchingly conventional at close hand.

He comes from what was evidently a close, happy family in Surrey and says he ranks his parents – well, now his mother Kathleen – as among his best friends. “I think parents are prouder of their children’s achievements than they are of their own,” he says. “My parents were lovely people, very kind and supportive even though our success was a long time coming. So it’s thrilling to be able to include them in everything.”

But the dubious nature of some of the Little Britain sketches meant the Channel swim conferred something even more precious than success – respectability. “My parents’ friends are in their sixties and a lot of them were like, ‘Little Britain, I’m not quite sure whether I approve of that,’ whereas, swimming the Channel, it was difficult for anyone to have a negative view.

“After I’d done it I got an invite to Buckingham Palace and I took my mum – now the first thing you see when you come into Mum’s house is a picture of her and me meeting the Queen. She’s got it right by the door as you come in so no one misses it. It’s been good being single for the last few years because it’s meant I could take her to things, rather than girlfriends.”

Which may have some truth to it, but paints a pretty misleading picture of Walliams’s glitzy lifestyle. He’s been pictured with many a gorgeous girl: Kylie, Geri Halliwell, Natalie Imbruglia and Patsy Kensit. Kate Moss is a friend. One interviewer turned up to find him ensconced with actor Rhys Ifans and his girlfriend, Sienna Miller. Walliams says he’s just a sociable type: “There are moments of glamour but my life is mainly spent in a room with Matt.”

But why shouldn’t he enjoy his fame? As he says, it was a long time coming. After Reigate grammar – where he remembers raising money for the first ever Comic Relief by performing comedy sketches for classmates – he studied drama at Bristol University. He met Matt Lucas at the National Youth Theatre in 1990 and they spent years slogging round the comedy circuit, putting on shows at the Edinburgh Festival and elsewhere, Walliams supplementing his meagre comedy earnings by working as a children’s television presenter.

Little Britain, with its eclectic cast of characters, started life on radio before transferring to TV. Walliams knew they had a hit on their hands when “every time there was a story about single mothers the papers started talking about a nation of Vicky Pollards”.

He and Lucas are about to head off to America to film a revamped version of the series featuring some new American characters and goodness knows what grotesques they will conjure up from trailer parks. Given just how English, never mind British, they are – they appear on their joint website in brollies and bowler hats – it’s difficult to imagine how they will adapt.

“People always worry about how humour translates but I just don’t see it as a problem,” he says. “Maybe our Britishness will be our main selling point in the US: Sacha Baron Cohen and Ricky Gervais are both huge.”

Swimming – he routinely does several miles a week – provides a respite. He uses the time ploughing up and down the pool or in sea training to dream up sketches “or just think about Beatles albums, films I like, whatever”.

Training and completing a Channel swim – and in one of the 50th fastest times ever – suggests a mental toughness at odds with his easy manner, but he says he was driven to do it by what he saw in Africa on a trip to an orphanage supported by Comic Relief, where virtually all the girls, aged 5-15, had been rescued from the streets, having been raped or abused.

He says ultimately he’d love to take over from Richard Curtis, the film director who heads up Comic Relief. It’s hard to tell whether he’s joking, but he says: “There comes a point when you think, ‘I’m sick of doing things for myself, I’d quite like to do things for other people’.”

To sponsor James Cracknell go to www.challengecracknell.com. For more information on Sport Relief and to sign up for the Sainsbury’s Sport Relief Mile go to www.sportrelief.com