Diana would be doubly proud as the organisation, which helps homeless youngsters, was one which was particularly close to her heart - she was their patron.
Dressed casually in jeans and a jumper, the second-in-line to the throne did the honour of cutting the cake, hours after he had served up breakfast at a charity hostel in Soho.
"Obviously my breakfast skills are horrendous," he joked at the time. "I wasn't allowed to cook an egg without an hour's health and safety briefing."
Inspired by his mother's work with the charity, Wills has been their patron since 2005.
He spend an hour and 20 minutes chatting with young people at the party. "It's much more intimate than doing a big place all suited up," he said.
"For me it's much more rewarding and I think that's because there's a lot more feeling to it."