Maxthedog, I bought my TF from a well-known dealer. WHen I drive it home, the MIL came on.
The dealer sent me a replacement ECU which had the MIL light switched off. (Symptom solved).
The dealer had installed a new CAT.

Recently, I had the Kmaps remap done by Mark (a brilliant improvement to driving and everything about the car).
Mark was the fuel and emissions engineer at MG-Rover when these cars were being built. He wrote the book.
(And a nice bloke too).

Mark explained that aftermarket CATS will cause the MIL to light and it is related to the design of the CAT.
The CAT will pass an MOT (although make sure you take it for test when fully warm or hot).but will not extinguish the MIL.
It may be possible to buy an NOS CAT from somewhere (maybe Rimmers) but at many hundred of £s compated to less than £200 for the aftermarket.

It looks like switching off the MIL in the ECU is the only option with an aftermarket CAT.
I think it is related to the distance between in and out O2 sensors on aftermarket CATs but I cannot confirm it.

I do recommend calling Mark to answer all your questions.
I believe the above to be correct but welcome learning more if it is not. And good luck.

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