Has it been driven much, cat failure is a much over reported item and it could be anything else.
Are the ignition components in full working order, if its been laid up, some corrosion may have occurred in the distributor and causing it to misfire ever so slightly causing the Lambda to richen or lean the mix depending on what it sees.
Any leaks in the exhaust, note: everybody says there isn't but a smoke test on all the joints will show if anything is blowing or your hand. But the exhaust gets up to temp quick so watch out using the hand method.
Is the Air filter clean? Sounds simple but it all adds up.
Has the fuel filter been replaced, and how fresh was the fuel? We had woeful trouble starting a chainsaw this year till we realised we filled it with last years fuel rather than the can we had only bought
Fuel quality has changed over the years.
The problem is getting it tested once you have done that simple stuff, you either find somebody with a proper 4 gas analyser to test it. Or send it in for the NCT test again with crossed fingers.
Bit of work in doing all that I know, but the worst will be that the car will be running great if it does turn out to the the Cat at least you have ruled out everything else. Replacing the CAT would be a good bit of work without access to a proper garage setup and may cause you more problems than solve them.
Start with the simple stuff and work up from there. Replacing the cat is a pig of a job.