Unfortunately it is VERY difficult to get your money back form a dealer wihtout resorting to the courts. They offer a 3-month warrenty which isn't worth the money it's (not) printed on. I found when I contacted the dealer who I bought my mare from that when you mention returning the horse under the warrenty the other suitable horses are suddenly more expensive or they try and fob you off with something they can't sell to anyone else. AFTER I bought my horse from this supposedly reputably dealer I started hearing stories about them - one where someone paid another £4,000 for a replacement horse after the one they sold him dumped him on a hack and he was taken to hospital. So they made £8,000 from this poor bloke. it sounds like the dealer you bought from NEEDS taking to court though - he's endangering lives! 

They are covered by the Sale of Goods Act but you'd need a solicitor and probably an expert witness. I don't think there is such a thing as a reputable dealer unless you're dealing with the seriously upmarket, serious money dealers.

I certainly wouldn't go back to a dealer in the future. They know exactly what they are doing and how to keep your money!