British riders took the top four places in the class, with Matthew Sampson (Eagles Costalot), Jay Halim (Goodman For Fun II) and Henry Turrell (Billy Comme Ca) finishing second, third and fourth respectively.
The riders had to jump a timed course of fences immediately followed by a row of ascending upright gates, and the stallion Billy Mexico relished the challenge. "You've got a chance of winning at every show you go to with him because he's fast and very careful," said William, who is based near Dorking in Surrey.
William won the Hickstead Derby in 2006, 2008 and 2009 - and this year he has a new Derby hope in Billy Onslow. The pair qualified this morning in the Bunn Leisure Derby Trial, which determines the starting list for Sunday'sEquestrian.com Derby.
But the honours in the Derby Trial went to Sweden's Angelie von Essen, who won on Jordan II and also finished third on Deesse VD Donkhoeve. Six came forward for the jump-off, with every rider faulting - which meant last to go Angelie was able to produce a slow but clear round to finish on one time fault and take home the top prize.
"I was quite insecure about whether he'd actually jump the course as he's green and inexperienced - he was only jumping Foxhunter last year," said Angelie.
Despite winning today and clearing Derby fences like the water jump, the palisade rails and the Devil's Dyke, the Swedish rider won't be jumping in the Equestrian.com Derby on Sunday. "I'm too scared!" she said. "I was nervous enough doing this class!"
Tomorrow's feature class is the Bunn Leisure Speed Derby, where riders have to tackle some of our permanent fences at top speed.