From the onlime MOT Manual
http://www.motuk.co.uk/mcmanual_620.htmSilencers fitted as original equipment
(i.e. are marked with manufacturers symbol) may not have BSAU or European approval markings, but are acceptable
2. With the engine running assess the effectiveness of the silencer in reducing as far as is reasonable the noise caused by the exhaust from the engine.
3. On machines first used on or after 1 January 1985 check that the silencer has an identification mark or plate-
Reason for rejection:
2. A silencer which is in such a condition or is of such a type that the noise emitted is clearly in excess of that which would be produced by a similar machine fitted with a standard silencer in average condition.
3. A silencer fitted to a motorcycle first used on or after 1 January 1985 which is not marked with either:
a. the make and type specified by the motorcycle manufacturer (See info column).
b. British Standard classification BSAU 193 or BSAU 193a or an EEC approval number prefixed by a small 'e' marking or an ECE approval number prefixed by a capital E.
c. A silencer marked 'NOT FOR ROAD USE', TRACK USE ONLY' or similar words.