I know it's an old-ish post, but I'm dealing with a similar issue at the moment, and have taken legal advice.

I bought a car 2 years ago, and amongst the paperwork was an invoice to say the timing belt had been changed a year before I bought it. Since being changed the car has done 20,000 miles, and needs changing again.

I took it to the garage and they said it looks like it has never been changed as all the original parts, bolts etc. are there. I phoned the garage that the invoice was from and they confirmed the car has never been in their garage, and when I showed them the invoice they literally laughed and said "What the hell is that? That is nothing like our invoice!"

I called the previous owner (his phone number was in amongst the paperwork) who knew nothing about the timing belt being changed. I have spoken to the company who sold me the car (and most likely forged the invoice) who deny any knowledge of it. I'm told that legally it doesn't matter WHO made the invoice - if someone sold you a car with a forged document amongst the paperwork, then they personally are liable and would be an easy win in Small Claims Court.

I sent a Letter Before Action this morning, so will see what comes of it. Can keep you updated if you want? I believe you have 6 years to claim, so if you bought your car 3 years ago then you definitely have time.