I was stopped in a layby for a roadside emissions test a few years back. I had just pulled out of work and the engine was cold... so told the bloke who basically said "Whatever". He then went to stick the sample pipe up the exhaust, and realised I didn't have one
It had fallen off the previous week I was sent on my way !!!
VOSA now do the roadside checks. http://www.vosa.gov.uk/vosa/publications/roadside%20emissions%20check%20leaflet.pdf
If you get emissions checked and fail you have 14 days to get it fixed and get it retested at an MOT station, or else you can be prosecuted.... So just srick the cats back in and get it examined, then take them out again.