I am sure Gimcrack wasn't doing that, though I confess I don't know.
My question is; what bearing would that have on the matter?
If a punter places a bet which wins, they must be paid out. A bet is a contract between the punter and the bookmaker and, if the bet wins, it is enforceable in law. Any other bets on the event, be it a horse race, football match, casino game or whatever, do not affect the bet which our punter has placed. It makes no
odds if other bets were placed by the punter's wife, sister, friend, enemy or Uncle Tom Cobbley. Each bet is a separate contract.
Gimcrack, my advice would be to take legal action against William Hill in the small claims court. The law which governs their business is that of England and Wales. You need to give them the chance to resolve the matter before going to court so you should first write to them outlining the circumstances. If the facts are as you have stated, then it seems that they have not paid out when they should have and, if so, they have no defence in law to what is a clear breach in contract. They may think they have, and there may be something in their temrs and conditions on which they will try and rely, but my belief is they will be unsuccessful.
They may also be guilty of theft by witholding money from you as this is an appropriation.