Ah, yea. SB are good for decent odds etc.
Betfred, Coral, Ladbrokes, W Hill and Bet365 all offer double the odds on 1 winner on a L-15. If you like having a few Lucky 15's over time, then one of these others might be worth signing up to for that purpose.
*BetFred i've noticed are robbing sods when it comes to anti-post prices before races, however, if you're happy to risk/go with the 'SP' then it's usually okay - Unless you know the horse you're backing is going to shorten.
Fwiw. BetFred are currently offering £40 in free bets when you open a new account and stake £10. Sign up using WELCOME40 if that is any use to you mate.