Normally a good hard thrashing round the block immediately prior to the emissions test is enough to get most cat equipped cars through an MOT emission test which they have previously failed.
It's just a matter of getting them hot enough to "light" and stay lit, as they have a habit of cooling down and stop working during the natural idle test.
That is presuming the engine is running ok - most of the time it fails is because they aren't (or they are Rovers
)