They are trying it on to save themselves money.
Be polite, speak to the manager and write them a letter outlining the problem. If they still do not respond, then write another letter telling them that you will be taking it to "an expert" at your own expense. Should the expert find the manufacturer at fault, you will be taking them to court for the cost of reinstating as well as the cost of the expert.
If you purchased your car on credit (either a Hire Purchase - not loan - agreement of if you used a credit card - not debit card - to place any kind of deposit of over �100.00, then you can also get recourse from the credit company.
You may also want to inform your local Trading Standards.