Under the Consumer Rights Act goods purchase from a dealer must be of satisfactory quality, fit for purpose and as described although a private purchase, they don’t have to be fit for purpose and they don’t have to be of satisfactory quality but they must still be as described.
I appreciate that it was only 3 years ago but I cannot see why you should not have a claim bearing in mind that under the Limitation Act, the time limit is 6 years.
How much to claim for?
The difference between the value of the car with leather and the value of the car with vinyl.
Don’t expect the dealer to pay up even with the threat of court proceedings and you are likely to end up in the Small Claims Court.
Gather your evidence with regard to the difference in price and then write to the dealer asking for their proposals and telling them what you want. Give them a deadline.
The new Moneyclaim beta system is easy to use: https://www.gov.uk/make-money-claim
the new system is much better than the old government Gateway system.
You can prepare the proceedings now, at your leisure, save them, and then just pay the fee and press the button when the deadline in your letter before action, expires.
It's then just a case of following the court process and responding to any emails that you receive.
Thank you for letting me assist you with your legal question. I am glad that I was able to help.
I am not certain whether that answers the question for you or not, but I am happy to answer any specific points arising from this.
It will be my pleasure to help you again either further with this or any future questions you have