That's a pity. The back-story here is that the car was sold without a catalytic convertor - just a piece of pipe bolted in place instead (at least, that's what I'm trying to prove as the EML light was on when I bought it)
Was it discovered at MOT time, perchance? You may have have a reasonable excuse to have not discovered the fault within the six month cut off. The dealer may have bought the car in like that, but the fact remains that they sold an unroadworthy vehicle.
How long before purchase was it MOTed? You said that you reported the EML to the dealer, what was their response? Make sure everything is written down with dates, even if you have no evidence.
I think you would have a case in small claims court with a report from the MOT garage, especially if they can give some indication how long since the work was done. It's fairly obvious if exhaust parts are recent, or if they've been on a year. Regardless of the 6 month cut off, SCC will judge on the balance of probabilities, and even if you have no evidence of what was said, it's your word against the dealer's. If you have a plausible log of what was said and when, and the dealer simply denies it all, it doesn't look good.
At the end of the day, it's down to whether you think it's worth the effort. There's no reason why you couldn't suggest to the dealer that SCC will be your next course of action.