If you believe the car was right at point of sale you should stand your ground. Admittedly everything's not black & white and any trader knows cars can sometimes bring up faults so if a punter phones up two days later to say his car is suddenly running like a bag of shite, then fair enough.
However these chancers phoning after a few months should be firmly brushed off.
The CRA can state what it likes & as for people threatening you with the court service, TS, etc. I always tell them to go ahead. Call their bluff, 99 times out of a 100 that's all it is, bluff.
On a sideways note I seem to get a couple per year who phone up 11-12 months down the line threatening court etc. because their car has failed the MOT on trivia like drop links or brake pads etc. and then quote some ridiculous figure they'll settle for (£1100 for Forester discs & pads - I suspect I could get a Jumbo Jet freshly shod for that money). My response is "Crack on, and its VOSA you need to speak to, goodbye"
Unfortunately 50% of the general public are as thick as pig shit & are horrible, lying, greedy bastards. Sorry if that sounds harsh but I genuinely believe this.