Ah yes, missed that because it wasn't the OP that posted it.
However, I'm still not sure there's a case here. The V5 states the title of the car, and that's exactly what J17 put in their advert. Have they misrep'd the car? Well, possibly, but they're going to argue that they sold the car exactly as described on the DVLA system and nowhere have they said it has 300bhp or 300ps. I'm not saying it's not shady, but neither is it clear cut and a good lawyer could get them through.