REJECTING CAR as NOT of SATISFACTORY QUALITY?

Successful rejection for breach of the condition as to satisfactory quality, by reference to the Sale of Goods Act, would appear to be extremely unlikely. Although allowing the seller a chance to repair does not in itself amount to acceptance, it's almost certain that using the car for 2 1/2 years and 50k miles would amount to acceptance.

However, if the car is on Hire Purchase or Conditional Sale, the situation may be different, as the rules on acceptance don't apply. Instead, you would be looking at whether the contract had been affirmed in the knowledge of the fault(s). Termination of an HP contract for breach on the supplier's part is in theory possible for a very long time after the contract is made -- although the remedy would not necessarily include a full refund.