A friend did after the vetting blood tests came back positive for NSAIDS (horse was lame when got home) however it was a very expensive and nasty court battle and dispite having some of the top vets in the country as witness for her - the judge was quite clueless and the dealer walked away with a slap on the wrist and nothing else.
I am not saying don't go for it - just be very prepared and pay for the best legal help you can! My friends insurance paid but not for a equine laywers which was also disastrous!
small claims is an easier option (and cheaper) but it has a limit of £5000. you don't need a solicitor and the forms have lots of helpful advice. Also trading standards are worth speaking to if the seller is also a dealer.. I know of two people who have successfully won a case in small claims court against dealers
A friend of mine took a very well known and supposedly reputable dealer to court after the expensive horse she bought from him turned out to have advanced DJD and simply wasn't as described in their advertising or on their website. It was very stressful and very unpleasant (extremely abusive phone calls from said dealer) but my friend stuck to her guns and wouldn't back down. She couldn't got through small claims as the horse was worth more than £5k so would have gone through the full county court system. In the end the dealer settled before it actually got to court (court proceedings were issued and he finally realised she was serious about going all the way). She got most of her money back but had to pay her own solicitor's costs, which came to around £1k by the end of the process (whole thing took over 6 months from beginning to end). She was also made to sign a confidentiality agreement preventing her from going public with the name and details of this particular dealer.
It was a very stressful time for her but she won in the end - so it proves if you have a good case you can win but you have to be prepared to really stick at it and find the money to pay solicitors fees.
someone tried to take us to court as a private seller, cos 4 motnhs after they had the poni it bucked their child off, undoubtidly they lost, the judge made a complete fool of them and they were very pissed off