Old fashioned letter sent by Recorded Delivery.

Proof they received it and keep a copy of the letter you send in a File. Correspondence can be wearisome but yu are wanting a refund on faulty goods and sometimes it takes time.

There is the 30 day right to reject under section 22 and the six months final right to reject under section 24. For the latter, money can be dectucted based on the use of the vehicle but not for the former.

Your right to reject the car under the CRA (Consumer Rights Act) lasts for 6 months, not 30 days.

Demand a full refund of what you paid. Do it in writing or email.

If the trader refuses, immediately contact the Trading Standards Department covering the locality of the dealer. Job done.

Trading Standards will have little interest in a civil case which this is. They'd only usually be interested if the car is dangerous, missold etc.

In truth your right to reject on a 14 year old Clio is very short and needs to be something very serious. In this case I think it would apply but don't go buying 14 year old cars thinking you have 6 month rejection right for much at all. You haven't.