VOSA do spot checks on emissions levels.
If you fail, you need to re-sit an MOT and go to VOSA H/Q to get it signed that you now have a valid CAT.
yeah, done loads round here. But i dont use my car everyday now. Also the modified car schene is big round here and many web based clubs local to me so news of the locations of the spot checks travels very quickly and car can stay in-doors!