C - Complain
Complain - It may seem terribly un-British, but if you have a problem with your car and it is not resolved to your satisfaction, complain!
Put everything in writing and detail every step that has led to your complaint. Start with the Customer Relations department, if the company has one. Talk to the most senior person you can at the dealership and send a letter and copies of any supporting documentation to a senior representative.
If that doesn't work, seek further advice from your local Trading Standards office. Always be calm and polite. Be reasonable but do not accept anything other than a complete rectification of the problem and any due compensation.
If you need help with a dispute you can't resolve, contact What Car? Helpdesk.