Presumably this is the seven-speed longitudinal s-tronic. That needs a change of fluid and filter every three years or so. If the BMW dealer filled it with normal ATF, then that is the problem. But anyway, you didn't, so liability rests with the dealer who sold you the car, even though it's outside the six months when a dealer is automatically liable. Hit the dealer who sold you the car for the £10k transmission replacement cost, or have him buy the car back from you for its current market value. Better the former because the county court small claims limit is £10k.