Is there any way you could get a second opinion, although appreciate may be diff if car sat there and unable to move it. From a legal perspective, the work the garage did should have been carried out with reasonable care and skill, and I guess it is for you to decide (perhaps with help of second opinion if poss) if this has been the case or not. If not then you are entitled to pay the garage a reas amount for the work they have done, taking into account the costs of getting it put right.
The counter argument is what the garage are saying, that if the whole system had been changed then it would have been OK. Have you got any proof they were wrong, or just a feeling? Feelings are very hard to back up and prove.
I can't tell you what to do, but hopefully a quick rundown of the legal position has been some help?
If in doubt, call your local Consumer Direct 08454 04 05 06