There was a great deal of confusion over the (Jan 2012) changes in the MOT even the testers were confused and required "clarification", and the amendments looked like a "u- turn" to me
it may be as time goes on the changes are phased in/ the MOT tightened up, things that were "advisory" may become "compulsory" in the future and the test becomes tighter
the official line is that it is to make the UK more in line with the EU - but my "gut feeling" is its to force older cars off the road (as the MOT gets tougher it becomes more and more difficult (and expensive) to MOT your old car there will be a point where it is more cost- effective to just buy a newer car)
I thought i needed a catalytic converter on my diesel car (late 2011), i was running a stainless steel decat,(2-1/2") heatwrapped, and heatwrapped downpipe, straight- through exhaust with "straight through" middle silencer, (the silencer is in the middle for better traction & handling balance compared to having it behind the rear wheels on a FWD car) and the back box deleted (replaced with unsilenced pipe) (all 2-14" apart from the decat)
its a mk3 Mondeo deisel
I had a EGR delete and a tuning box (i took the tuning box off and reset the ECU for the MOT though)
The car sailed through the MOT (and emmisions - really clean exhaust output) in 2012 i spoke at length to the tester and he said the decat was no problem - only the particulates are measured - there are covers on top of the engine, a heatshield on top of the cat/ decat (but you can see it if you look hard enough and know where to look) and an undertray under the engine, Testers are not allowed to remove covers
The car was a bit louder than stock - but that was not a problem either (testers discretion?)
in 2013 i thought i may have a problem but again, the decat was ok , failed on windscreen and rear middle ceatbelt (i never use it so i removed it to save weight, along with a few other things) the tester seemed quite impressed with the car - i "picked his brains" and asked a lot of questions, he suggested i just take the back seats out, a brand new windscreen later (heated - don't ask the cost (ouch!)) and missing its back seat i got my MOT certificate in my grubby paws - the back sats weighed a ton (27kgs) so it was just the excuse i needed to leave them out and strip out the back of the car, black out the windows and convert it into a van - with all the lightweight parts and stripping it took it down to a healthy minus 150kgs
Aldoo, - technically the best thing is to cut off the cat, and weld a straight bit of pipe in its place, the same diameter as the pipes with no steps or bumps, if you leave the cat on even with the insides not in, you will lose velocity (flow/ power) but it would be better than having a full cat, though
I can't remember if it was my idea, but you could build up a lot of heatwrap in a fat "roll" untill the decat/ straight pipe looked like a cat that had been heatwrapped, from the outside, if you know what i mean (i was planning this, but it turned out i did not need to)
Thanks about the comment about my car - ive not done much to it recently scince the 3rd superchips remap and the electric waterpump - ive got a load of plans for the summer/ when the waeather warms up a bit (if it ever does 
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