This was photocopied out of the MOT testers handbook!
Page 5 of "The MOT Inspection Manual" Issue Date Mar '92
Please check it is still current!
A high definition version of this scanned page as a JPEG VIRTUAL opened from my server and you can read the full emissions testing procedure
Exhaust Emissions 6.4 Page 5 - Method of inspection - Col2 Para2 reads..... For vehicles registered on or after 1 August 1975....."If A vehicle meets the CO requirement at normal idling speed but fails the HC check, re-check the HC level at a high idle speed of around 2000 rpm. if the HC reading is then 1200 ppm or less, the vehicle should be considered to have met both the CO and HC requirements."
This means that owners of cars with engines that are running "off cam" at ordinary idle speeds, (like MG Metros) can ask for the HC (Hydrocarbons)test to be repeated at 2000 rpm - the engine must meet the CO output at ordinary idle speed (this is easily set) but the HCs are almost impossible to control at the bottom end with longer duration cams!
I believe the above modification to the MOT procedure was actually introduced to get MG METROs through the MOT as they do run a hot(ter) cam than standard