I agree with Dunk, I think you are going to have to go back there and prove it too them once and for all, I would recommend going there on a saturday afternoon when its nice and busy so you've got enough spectators, make a scene, make it be known that you've got a problem with this place but be cool.
I see your in wales direction a bit of a drive maybe there's a supra owner out birmingham ways who would be willing to meet you there, that way you can drive down with the wheels in something a bit more economical and not waste any more money on these guys. Once you show them whats wrong then there is no dispute, I'd take a camera aswell and take some pics once they've put the wheels to have a record.
Just don't take no for an answer