David Walliams on his Big Swim for Sport Relief

But Big Dave – he stands 6ft 2in – is not a man to give up. When he was considering what to do for his third charity swimathon, he hit on the Big One: front-crawling 140 miles down the River Thames.

It meant bone-deep fatigue, freezing temperatures, hazardous boats, wetstuit chafing, sewage, a torn spinal disc, parasitical infections, drowning dogs, errant shopping trolleys, basking Antony Worrall Thompsons, irksome Rob Brydons and exploding microwaved porridge served up by a well-meaning but klutzy Miranda Hart.

“I don’t know about you,” says Walliams with a tut and a faint arch of his eyebrow, “but when I make porridge I use milk and honey. Not water, and certainly not from a sachet. Not good enough…”

Still, even for a man with Walliams’s experience of wildlife and nightlife, catching “beaver fever” must have come as a shock. “I got this thing called Giardiasis,” he says, using the medical term.

When he was still suffering from an upset stomach a few weeks after completing his aquatic odyssey in September, a specialist diagnosed the condition and prescribed antibiotics. Luckily, by then he was off the painkillers he’d been taking to help his aching limbs during the swim – although it was only after they wore off that Walliams discovered he’d injured a disc in his back. He still receives physio for that now, five months later.

Giardiasis is quite common in the Third World,” he continues. “Obviously I took on quite a lot of water because it’s just impossible not to. When a boat comes past there’s a wash. And when you turn your head to breathe, you get a mouthful.”

Did it taste… well, a bit like sewage? “Mmm,” he muses, “it tastes kinda earthy. Because you’re used to the taste of swimming pool water, and to a lesser extent the sea. At the river’s source, in Lechlade, it was actually quite nice because the water comes from an underground spring, so it’s quite pure. But by the time you get into central London, it stank.”

And did he also encounter the man-made flotsam and jetsam of old Mother Thames?

“You know what, the visibility is so poor that you can hardly see a thing. At certain points I could see the bottom ’cause the water was only 5ft deep in the shallow bits. But once you got into central London, past Teddington Lock, there was zero visibility. So that took a bit of getting used to. It’s an unusual environment to swim in.”

The journey, chronicled in the documentary David Walliams’ Big Swim, sees him visit a centre for orphaned street kids in Kenya, and make friends with 12-year-old Philip. Back in London we see cheering supporters lining the riverbank, a walk-on/dive-in appearance by Labrador Vinnie (Walliams helps to rescue the flailing pooch), and cameos from chef Worrall Thompson (who happens to be boating by) and Rob Brydon.

“When I helped Vinnie out of the water I had no idea it would become such a defining moment of the swim,” says Walliams. “I think we British love animals so much, by saving Vinnie it made me seem even more heroic. It was a simple act that raised the team’s spirits far into the swim, just when we all needed it, especially me.”

Welsh funnyman Brydon hogs the camera and pesters his friend. Be honest, I ask him: was Brydon a help or a hindrance? “Oh, definitely a help. But he was a tiny bit distracting. He was really going for it – ‘I’m in this documentary, so I’m gonna contribute and be funny…’ I could hear him saying, ‘David! I want to ask you a question!’ ‘I’m trying to swim, mate!’”

The “ickiest” bit, he says, was dealing with nausea on the third day. “Oh, it was the worst,” he sighs. “When you’re feeling that sick, the last thing you wanna do is go swimming.” His problems were compounded by the water temperature. He’d been assured by his trainer, Greg Whyte, a silver medallist at the pentathlon and Professor of Applied Sport and Exercise Science at Liverpool John Moores University, that the Thames would be about 18°C (a pool is heated to 24 or 26°C). But after the cool, wet summer, the water was 15°C.

After Whyte spotted him turning blue on the first day, Walliams donned a wetsuit, scuppering his plan to do the swim clad only in trunks and naked determination. “I thought people would think it was too easy if I wore a wetsuit. It’s one man, swimming trunks, hat, goggles – I thought that would be more impactful.” But even if you weren’t feeling nauseous, “Cold water is just not a very safe environment to be in.”

Did he actually vomit? He shakes his head. “I had a really upset stomach at the… other end. It’s a difficult thing to talk about.” Of course.

Did he “do a Paula” [marathon runner Radcliffe, who famously squatted roadside mid-race]? “No, I didn’t. But I did just want to lie down. So yeah, that was tough. But in retrospect it was good it happened – I think it made more people interested because suddenly there was jeopardy. A sense that ‘he might not make it’. And more and more people came out to see me.”

Did he fear for his life? “Uh-uh,” he demurs. “I was very well looked after by safety boats and a great team of people. But the biggest danger was being hit on the head by a boat. That’s how a lot of people drown.”

Why put himself through that? In the documentary, stand-up and celebrity chum Jimmy Carr suggests that his friend suffers from self-loathing.

“Um, yes, I’d say there’s a degree of it,” replies Walliams with only the slightest hesitation.“Those kind of things are set off in childhood, aren’t they? I mean, I still enjoy my life… And I feel like I’ve achieved enough things that if I never did anything again I’d feel confident that I’d still made my mark in some way – creating a very successful TV show with Matt and writing some children’s books and having done some acting. But maybe the self-loathing bit is the element that makes you strive for more. Makes you strive to be better.”

On his previous challenges, it was one, unattached man against the elements and his fears. But now he’s half of a married couple. Model Lara Stone is visibly upset in the documentary when she sees her bleary, puffy-faced husband emerge from the dirty river. Did her worries make it harder?

“Well, I didn’t like to see her upset, but it was great having her support. She actually had her driving test on the day I got ill, so she heard it on the radio and was doubly upset she hadn’t been there to look after me. But she passed – first time. I passed third time, so she holds that over me.”

Stone says at the end of the programme that Walliams must never do anything this dangerous again. After all, they want to have kids, and soon (“I’m already old for a new dad…”); they even briefly discussed adopting Philip, the boy Walliams had bonded with in Kenya, before the impracticalities became apparent.

So have they had conversations about other challenges he might undertake? “Yes – just after the Thames swim we went to New York. Lara had to go for fashion week, and I wanted to get away, I’d had so much attention. And I’m friends with Bear Grylls and he was there too.”

The adventurer wanted to discuss a new endeavour with Walliams. “It was only scaling a rock face, but Lara got really upset and said no…” Luckily, Walliams’s Giardiasis flared up and he had to cancel the meeting.

Still, he has loads on his plate: a role in the big-screen version of Great Expectations; a script he’s writing for BBC1 of his book Mr Stink; a possible play with Timothy and Rafe Spall and Sir Trevor Nunn; a school-set sitcom; a documentary on Roald Dahl; and, right now, a berth on the panel of Britain’s Got Talent.

“Well,” he says, clearly having given some thought to this. “I jumped from 22 miles with the Channel to 140 miles in the Thames. So to leap from 140 – the only thing I can think of would be 3,000 miles.” That would be the equivalent of splashing his way across the Atlantic.

“I think it’s been done, but it took about six months. And to me that would be misery. Also, in the middle of the ocean, miles from land, you’d feel really vulnerable. That’s a scary thought. “And I’d worry that by the time I got to the other end, I wouldn’t be famous any more. So much time would have passed, everyone would have forgotten – ‘Ooh, who’s he? It’s years since hewas in Little Britain…’” And much as he loathes himself,David Walliams would hate that much, much more.

David Walliams’ Big Swim: a Sport Relief Special is on Thursday 9.00pm BBC1