Its an interesting concept, but of course under UK law, your dispute is with the supplying dealer not MMC. If this action went to a court of law, it would be the dealer in court not MMC. The dealer's lawyers would then get MMC involved as they try to pass the buck to the factory. The dealer is really caught in the middle, the owner is obviously peeved about his poor M3W, and the factory do not appear to be taking positive and timely action to solve the problems. Of course being remote from the communications between dealers and the factory, one can only speculate.
Neil