David Walliams talks to TV&Satellite Week about his epic swim along the River Thames last September in aid of Sport Relief

Why did you choose a gruelling 140-mile swim down the River Thames?
“I chose the Thames because I thought it would be an amazing challenge. I thought it would capture people’s imaginations, and I know how people like to see people off the TV suffer.”

You did the challenge to raise awareness of street kids living rough in the city of Kisumu in Kenya, where you were particularly taken by the case of one boy, 12-year-old Phillip. Can you tell us more?
“When you’re told that there are 20,000 kids living on the streets of Kisumu, it’s hard to process. When you meet them individually it’s much more affecting. I’ve seen how the money raised helps people and I owed it to Phillip and those living unimaginably tough lives at home and around the world to keep going.”

You clearly are determined to see these physical challenges through to the end?
“Because I’m quite effeminate and don’t look particularly fit, it comes as a real surprise to people that I’ve got this kind of resolve.”

You were struck down by a stomach bug on the third day of your eight-day swim. That must have been terrible to cope with…
“When you’re feeling really sick, you should be lying in bed with your mum bringing you Lucozade and Rich Tea biscuits. At no point does your mum say, ‘What you should really do is go swimming in a river.'”

You had a lot of support, including your comedy chums Rob Brydon, Miranda Hart and Jimmy Carr, plus crowds of well-wishers cheering you on. Did that lift your spirits?
“I’ve never had such appreciation. For people to come out just to see you do a bit of swimming, get out of the water and wave is extraordinary. It felt like a really British event, with people cheering and waving their Union Jacks from the banks of the Thames – a sort of one-man amphibian Royal Wedding!”

David Walliams’ Big Swim: A Sport Relief Special is on Thursday, BBC1 HD,9pm