Arcturus
This seems quite simple to rectify. Send another letter to the garage, again recorded, stating that you did not agree to this work being done. Give them the opportunity to either:
1. Reduce the bill to the previously agreed rate
2. Refit the original part and refund your money as you didn't agree to the work being done
3. Quote you for the work and you can decide whether or not to accept their quote
If none of their proposed remedies are to your satisfaction then inform them that you will pursue them through the small claims court to recover the money in excess of the sum you were originally quoted to do the job. Be clear that you are giving them 7 days to respond to your request.
Small claims is not a scary procedure and is normally, if not always, done without solicitors. The cost is small, you just need to contact the court and get a claim pack. All of the details of what you need to do are contained in the pack. Don't let these arrogant garages get away with ripping people off and then holding your car to ransom. This is verging on extortion.
Please keep us informed as to how you get on.
