Blocked cats / induction leak

[jamesuk28]

Still have a very rough engine when hot. Cold is fine. The usual has been replaced / checked to include ignition amplifier / stepper, its had a full diagnostic check and tune. The valley gasket was not sealed correctly, however my mechanic (and he knows his stuff) does not think that this is 100 % to blame.

Could the cats be blocked? he said I can decat very easily and it should still get through the MOT. I thought (the car is a 1993 Chim by the way) that all cars made in or after 1993 required a cat.

Any other ideas? Injectors perhaps