Local resolution 1: Complain to the manager of the garage directly. We have included links to some very useful information below on dealing with the car industry.
If you have to pay a bill before making a complaint you should write a letter to the manager detailing your complaint, saying you are not satisfied and that payment is made without prejudice. This will protect you if you want to escalate your complaint or need to go to court.
It is important that you make any complaint as swiftly as possible as it may be hard to prove a problem if you have used your car extensively since the problem arose.
If you are unable to resolve your complaint with the garage you can go to step 2 or to step 3 if there is no higher level within the company you can turn to.

Local resolution 2: You should complain to the head office of the dealership/car company involved if applicable. You should also check if the dealership is a member of a trade association such as the Retail Motor Industry Federation. If so you can often complain to them or establish the charters that they have for their members. Many offer independent arbitration services which you might consider but you may wish to take advice from step 3 first.

Authorities: In all cases, if the complaint is still unresolved then you can seek advice from your local Trading Standards Office or Citizens' Advice Bureau. They will give you free advice and may take up your complaint for you.

Court: Failing all else you can still go to court to resolve your dispute. Details of small claims court and other procedures are given in the Legal Section of this site. You can also contact the Community Legal Service (justask.org.uk) that gives access to free advice. Otherwise your local Citizens' Advice Bureau will give you advice and will sometimes organise free legal representation for you.