If you are looking for a garage which will treat you fairly and charge you reasonably, you can search the data base of businesses which have signed up for the Motor Industry Code of Practice.
It covers new cars, service and repair as well as warranty provision and operation.
There is a free post-code based search of garages near you, a telephone help line, and you can complain if you feel you have not been treated as you would expect from one of the businesses which has subscribed to the Code of Practice.