Dear Sir/ Madam
Ref: [registration number of vehicle]
On [date] I purchased, and took delivery of, the above vehicle [make and model] from you. On [date] I discovered that it was not of satisfactory quality: [describe the problem].
The Consumer Rights Act 2015 requires dealers to supply goods that are fit for purpose, as described and of satisfactory quality. However, the vehicle is clearly not roadworthy. You are therefore in breach of contract.
[If you've owned the car for less than 30 days] I am legally entitled to reject the vehicle and to be reimbursed the original purchase price of [£xx]. I look forward to receiving your cheque for this sum within 14 days.
[If you've owned the car for more than 30 days] I am legally entitled to ask that your repair or replace this vehicle at no further cost to me. If you are unable to do so, I expect to be reimbursed the original purchase price of [£xx], minus an agreed amount for fair usage.
If you fail to reimburse me, I shall have no alternative but to issue a claim against you in the county court for recovery of the money without further reference to you.
[Your name and signature]