Modified vehicle and catalytic converter(s)

Looking for some advice regarding tuned car that was purchased last year
Vehicle is Stage 3 modified and tuned and
At MOT just over 6 months after purchase it failed it's MOT emissions test and also on the absence of a Catalytic converter.
Vehicle was sold with a current 2020 MOT (but curiously no emissions test results) and the dealer provided the 2 previous MOTs including the emissions test results
The 2020 and 2019 tests were done whilst in the dealers ownership but 2018 was the previous owner. 3 different testing stations.

The car was taken to have a sports catalytic converter fitted and passed its retest the following week.

We're pursuing the car dealer ( who aren't denying that they knew the vehicle didn't have a catalytic converter) . Their defence is that the car was advertised and sold as a "Track car", however nowhere on any adverts we've retrieved do they mention that.
Further down the line of " enquiry" after they'd passed it to their solicitor, they also decided that we were also aware that it didnt have a catalytic converter ( ie that they knowingly sold us a non road legal vehicle and we knew that it wasn't road legal).They haven't tried to explain the passed MOTs .
From the dealers admission we know that the modifications to the vehicle were all carried out by the previous owner. We're also aware which company carried out all the modifications.

Having had the sports cat fitted and the car then passing it's MOT, we thought we were home and dry ( apart from trying to claim costs from the dealer), but from what reading , the lack of the original 2 cats might be a problem in the future.

Any advice appreciated