put everything from your side in writing as quickly as possible, check with Citizens Advice as they have a template for this kind of thing - you can start it with "as discussed with you in person on the xx/xx/xxxx"
make it clear what your issue is and what you would like to be done, make it clear that you want a written response within 14 days and that you want your car back either fixed or refunded within 14 days also (they may reply and say they will fix the fault for free but they should then give you a "reasonable" completion date)
if they come back and say that the repair they previously performed is "good" and that something else is now wrong with the car, you will need to take it to a 3rd party to get an "engineers report" to see if what the garage are saying is true
you can then submit the whole lot to your credit card and if the 3rd party agree with you that the garage did not correctly fix the fault, they should eventually refund your money - someone like the AA will be able to do this for you, another local garage are unlikely to want to get in the middle of an argument between you and garage A, but it might be worth a try
if they've ever given you a date for completion and not met it, then they are also in breach of contract for not completing the job in a "reasonable" amount of time, but again you need to get these things down in writing to be able to submit it to your credit card company