Betfair Sportsbook is the place to bet on the Grand National. Not only do Betfair offer a bonus bet deal for new customers, they'll also usually payout each-way bets on the Grand National to five places.
Along with the traditional fixed odds service offered by Betfair, new customers can also take advantage of the Betfair Exchange, where you may find bigger odds available on many of the Grand National runners. The exchange also allows you to back horses to lose! However, I'd advise novice punters to stick with the traditional Betfair sportsbook before diving into laying horses on the exchange.