They are a bunch of cowboys. The fact its been in an accident is irrelevant. Send them a registered letter informing them that the car was working when you took it to them and that they have a duty of care whilst its in their worshops and that since it will not work after their modification its their responsibilty to fix. Give them a deadline and inform them that if they don't comply you will get the car fixed elsewhere and present them with the bill going to court for the monies if they don't pay.

If these people are trying to say the fault isn't theirs, it's that the car was damaged, they should have refused to do any work on it.

By doing the work they have accepted responsibility.