Strictly speaking the car supermarket is liable because the fault must have been present or developing when you bought the car. Whether they take the car back or pay for the repair will depend on the Alfa specialist's quote. I don't think they will be liable for the timing belt and water pump unless they had previously agreed to pay for them or you could prove that they were overdue. See: www.honestjohn.co.uk/faq/consumer-rights/