they cannot do anything if the car has no cat fitted, as it is not a legal requirement, the legal requirement is that the emission fall between a certain limit, if you fail the test, then yes, you will have to correct the problem within 14 days and as jay says, you will be given a rectification notice, which means you go to an MOT test centre and they make sure the car is legal on the roads again, then you go back to the poice station and all is presnt and correct
if the above makes any sense to you