Further research ... the AA website
http://www.theaa.com/motoring_advice/car-servicing-repair/mot-changes-2012.htmlsays
"Speedometer
The car will fail if a speedometer is not fitted, is incomplete, inoperative, has a dial glass broken/missing or cannot be illuminated."
so implies that it is checked .. but I can't find it in the schedule ...
EDIT
although I now have something relevant
http://www.transportoffice.gov.uk/crt/doitonline/bl/mottestingmanualsandguides/mottestingmanualsandguides.htmHit "Inspection Manual for Classes 3, 4, 5 and 7 Vehicles" - Browse
then contents, then next page, then Section 6, then section 6.7 Speedometer
3rd column
"2. Speedometer incomplete,
clearly inoperative or the dial glass broken or missing."
so if the Inspector sees it is "clearly inoperative" ?? then he can fail it.
I guess if it doesn't move as he drives it onto the test patch, or during the brake checks .. it's "clearly inoperative"