Sorry to hear of the issue.
The issue is, the garage has your car and they have a lien - meaning they can legally keep it until you pay for the work.
You would have to either get a law firm involved to write to them and put pressure on, or you will need to pay under protest so as to retrieve the car, and consider legal action after that.
I don't know if the problems with the car are due to their poor work or if they are simply fixing the car which had faults.
If they caused this, you can demand a refund under the Consumer Rights Act 2015 though the problem remains that they have your car and can keep it until they are paid. You could do that and send a letter before action to them afterwards to demand payment of your losses and if they refuse, you can issue a claim - which is easy to do.
You could use a law firm - which may be wise if the garage is fobbing you off, which seems like they are from reading your question.
I can set out details of consumer dispute lawyers if that helps - I do have a template letter you can use if this helps.
You could also speak to Trading Standards to report the garage but they take a long time to deal with these disputes in my experience due to the sheer numbers of similar complaints they deal with on a daily basis.
Happy to help further if you would like to have a think and please let me know.