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 of satisfactory quality. However, the vehicle is clearly unroadworthy. You are therefore in breach of contract.
I am legally entitled to reject the vehicle and to be reimbursed for its full purchase price of [£xx]. I look forward to receiving your cheque for this sum within 14 days.
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]