If the catalytic converter is rattling due to it broken up, is it instantly an MOT failure? Or is it ok as long as it passes emissions? - I suspect this is the case, but it probably wont pass on emissions if its broken up?

I would expect that the test checks for exhaust system gas tightness, mechanical integrity and emissions.

If the cat is broken up there may not be enough of it left to work effectively...

it may still pass the emission test as what usually fails is the internal metal holders for the exotic metals used to reduce the emissions, but may be failed by the tester as is it not working correctly as intended by the manufacturer,what i did with mine for a temp fix was removed the cat and banged it on the floor to lodge the loose center part in the one end of the down pipe, this worked for about another 2 years ;).
but with now reduced prices and cheaper parts it is just easier to replace it while its off the car

instant fail my failed on it

As above, I would'nt fail for the rattle but It depends on the emissions.

Sure its not the heatsheild on the outside of the cat rattling? Common problem i had it on my old 200 sei. I just put a huge jubilee clip round it and it stopped.

removed the cat and banged it on the floor !!!!!!
Watch out for animals activists:bgrin:

Before changing the cat, get the car up on a lift and with the handle end of a large screwdriver push it upwards under the manifold pipes just before the cat. Sometime back there was a weld that corroded here and made the exhaust very noisy. I had 2 manifolds replaced on cars I had due to this fault.

I have been a class 4 and 7 tester for twenty years and as long as the car passes the emission test and the system is gas tight there is no reason to fail it.
If the cat core has failed and burnt down then it will not be working at all and this will fail the emission test.
If it is rattling simply because it has broken loose inside it's casing (usually the case) then this will not fail and can usually be cured by denting the casing to grip the core.

Check the heat sheilds to make sure that they are not causing the noise.

Appart from that cat units do not hold the mystery that they used to command and have come down to reasonable prices these days so don't be too worried, I remember the days when a failed cat could mean bills of up to a grand but those days are long gone so dont panic.

I will back up the heatshield option. Replaced my cat two manths ago because of the rattle, on inspection, it was the shield. Now after all this snow and car scraping on the mounds of ice between the tyre tracks on the road, i am hearing its return