There's your problem - William Hill are a shit bookie. Paddy Power/Betfair Sportsbook are offering 17/1. The draw is a best priced 7/1 at Unibet.
Oh well, perhaps I'm a shite punter too - they probably wouldn't shed a tear if they lost my custom - I put in a tenner to have a flutter on the National last year, and I've still got about two quid left in my account! I'm not going to start having multiple accounts - one's plenty for me!