And now the DVSA don't train testers, it is out sourced. They are purely there to catch garages breaking the rules to generate income in fines...er I mean to help garages maintain a fair and legal service . It used to be that you'd see one every 2-3 years, but if you live in an area where 3 garages near you are and amber risk and you're a green, while they're watching your neighbours, they'll be watching you too.
One way around it is to pay the tester the labour to swap the cats for the mot and swap em back after. He's covered and you get the mot. Of course you can still be done for having them on now, but that was always the risk.