The lengths some people will go to for charity

140 miles, eight days, freezing water, raw sewage: David Walliams plunges into his biggest challenge yet... as Sam Cam sets out on a one-mile walk through London's West End, equipped only with a pair of stilettos

David Walliams during his attempt to swim the entire length of the River Thames to raise money for Sport Relief (PA)

"I do everything he does, only backwards and in high heels," the late Ginger Rogers supposedly said of her rather more generously remunerated dance partner Fred Astaire as she argued with studio bosses over a raise.

Alas, the same cannot be said for David Walliams and Samantha Cameron, who embarked on two decidedly different fundraising initiatives of their own yesterday, despite the Prime Minister's wife's audacious, three-inch Top Shop heels.

The Little Britain star and English Channel conqueror plopped into the Thames at Lechlade in Gloucestershire, beginning an eight-day, 140-mile swim to raise money for Sport Relief, which, salmonella, E.coli, raw sewage, freezing temperatures and dangerous currents notwithstanding, will take him the length of the river all the way to the foot of Big Ben.

Sam Cam, meanwhile, met fashion designers at 10 Downing Street before leading the inaugural "Fashion Mile", a stroll to "raise awareness" for Save The Children. Fashion designers Sarah Burton, Christopher Bailey and Erdem Moralioglu joined her on the walk to Somerset House.

It is the first of what the charity hopes will be many sponsored "fashion walks" and other events across the UK as part of its Born to Walk Tall campaign. Mrs Cameron heard from Dr Hejara Kera about the challenges she faces in her work running a clinic funded by the charity in Katsina, a remote part of Nigeria. "Nigeria has a huge population and not enough health workers, facilities and equipment. As a result, women are suffering long and dangerous labours and sick children die needlessly. We desperately need more help," Dr Hejara said.

As the old proverb almost goes, you should never judge a fashion designer until you've walked a mile in their shoes. But it is not unreasonable to suggest Walliams may have taken on the tougher of the two challenges.

A short distance into the swim, his trainer Greg Whyte spotted a blue tinge on the comedian's back and instructed him to change into a wetsuit at the first stop.

"The first section of the swim has really shocked me," Walliams said. "The water is two degrees colder than when I swam the Channel, just 15 degrees. I'm a bit daunted by the way this has started, you can train as much as you want but with these kind of things you don't really know what it will be like until you start them."

His sponsorship pledges have already passed £65,000. When he swam the Channel in 2006 he raised in excess of £1m. This swim, he pointed out, is more than 120 miles longer.

Walliams is set to burn 4,400 calories a day swimming 17.5 miles. He was expected to end day one at The Ferryman Inn in David Cameron's constituency of Witney, Oxfordshire.