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Ah ok, didn't realise it was that clearly stated. I've only really got previous experience of cat modification, but even recently I've never heard of anyone actually fail the test on the part being missing or altered, only if emissions don't meet regulations.
3 years ago I had a 200 cell cat, tester was fully aware as we had a good chat about it. Tried a number of times to get it to pass, couldn't, so I put the standard cat back and stuck with that since.
UK? It's only actually illegal if it fails emissions isn't it?