£150 for a clutch isnt fair if you can get it done cheaper elsewhere. didnt you get quoted £80 elsewhere?
plus your gearbox does not have 6 other mounts. if a mount has broken, and the car is under warranty, do not accept it back unless the mount has been replaced or repaired.
if the mount was not important it would not exist in the first place. one broken mount puts extra strain on all the others, meaning they are then far more likely to fail. also the engine may now be able to move more than it should. you need all your mounts.
i wouldnt accept the car back. tell them to keep the car while you take the warranty paperwork to citizens advice.