There are a number of things you should know first is that 80% of garages are according to a BBC report are charging excessive repair and servicing fees.
In the London Area they are as high as £170 an hour and elsewhere in the country 80 % of garages were charging £100 plus, franchise dealers charged on average of £66.35 an hour and peaked at £117.45. The cheapest independent garages charged just £23.50 an hour for labour,does this mean that the more you pay the better the service not necessarily. Surveys into the standard of car servicing regularly highlight the following problems
Charging for work which is not carried out
Excessive differences between the final price and the initial estimate
Poor, substandard, unsatisfactory or even dangerous workmanship.
There are also garages with AA or RAC approval which will give you a bit of security but with all these garages you should take certain precautions unless you have a good relationship with them. Remember once you give them your car to service you will have to pay whatever bill they give you to get it back and if not satisfied with the work they have carried out negotiate or sue
You cannot refuse to pay garages they have the right to keep your car until you settle their bill and as said before you may have to sue for work that has not been done or was not authorised or badly repaired. Removing your car.stopping cheques or credit cards are criminal offences you should pay the bill and write on it under protest.
There are a number of things you can do to try and avoid problems
Make sure that the garage understands what work you require and preferably get something in writing that shows this.
Tell them what you want and make sure that they are able to provide it.
Find out the length of time the work will take and when you will be able to collect your vehicle.
Check what means of payment is required to settle the bill.
Leave a phone number or means of contact so the garage can ask you permission before doing any extra work.
Obtain a detailed invoice listing price of parts and labour costs individually.