Except for bednobs all the above answers seem to be wrong!
"Regulation 33 of the Road Vehicles (Construction and Use) Regulations 1986
Item 5.
There are no requirements for a mirror to be fitted to a two-wheeled motor cycle with or without a sidecar attached.
However, if mirrors are fitted to the above vehicle then it must comply with the following
(a) if the vehicle is first used on or after 1st October 1978, the mirrors must comply with
(i) item 2 of Annex 1 to Community Directive 71/127 79/795 or 80/780; OR
(ii) Annex II to Community Directive 86/562 or 88/321; OR (iii) paragraphs 4 to 8 of ECE Regulations 46.01
AND paragraph 4 of this regulation
(b) in any other case, no requirements apply.
For the purposes of this legislation, a motorcycle is defined as: A mechanically propelled vehicle, not being an invalid carriage, having less than four wheels and the weight of which unladen does not exceed 410 kg."
Afraid that this has been superceeded dependant on age of bike.
Any model of vehicle which was being sold in the UK before 17 June 1999 could continue to be sold up to 16 June 2003 if it complied with The Road Vehicles (Construction and Use) Regulations 1986, as amended (C&U) and The Road Vehicles Lighting Regulations 1989, as amended (RVLR). After 16 June 2003 European Whole Vehicle Type Approval applies.
www.vca.gov.uk/additional/files/vehicle-type-
approval/vehi...or-motorcycles/vca039.pdf