Just posting to see what other members do at mot time.
when i took it to be tested i had the origanal cat in the boot ready.
it came to the time for the garage to the emisions test so tried to fit the cat,fitted turbo end ok but due to new pipe being 4 inch would not fit to the rest of the system.
Luckally i have a good garage and they cut off the flange end and made a step up length of pipe and welded on put in the system and it passed,then put the de-cat pipe back on.
so now next year its just a quick swap round