Betfair.
As for people saying just take your money and run, why shouldn't he fight it? Refer the dispute to eCogra (
http://www.ecogra.org/ ) and get a ruling from them. You can also refer it on to the licence provider, which is the Gibraltar Gambling Commissioner. Present them with as much information as possible. Take any emotion out of your message to them and just present them with the facts in as succint a manner as you can. If you can provide evidence of the odds that other books were providing at the time, do it.
Good luck in getting your money.