I think you will find that all used car warranties exclude certain items - typically friction linings such as clutch and brakes, exhaust, battery.
It is near impossible for the garage to have known the condition of the clutch when they sold the car without dismantling it. The fact that it worked would have been acceptable although there may have been indications that it was worn.
How they are going to prove any abuse is where they will fall down. There is virtually nothing they could do to prove that and that is the issue I would be concerned about. OK they could dismantle it and claim it had been overheating but I doubt that.
If you a member of one of the motoring organisations I would speak them for advice. If not your local Trading Standards could advise.
Good luck and don't take any cr*p from the garage !