Go back to the start. This is actually quite simple to solve.
Paperwork, paperwork, paperwork. Get receipts, dates, times, conversations, letters, everything. Get and keep a record of everything you asked them to do, what they did and what problems you had since. Send them a letter via a recorded service (Special Delivery, not Recorded Delivery!) stating a reasonable period of time (7-14 days) for them to either fix the problem ir you'll take it to court. Be very sure in your paperwork here and you'll be fine: You take the car somewhere else, get them to fix it and claim the costs back from the garage.
Again, be very sure here as otherwise you'll trip yourself up in court. Don't falsify stuff or blag it a bit, be honest and truthful and you'll be fine.