A copy of 2 posts I just made on Ducatisti related to this
Taken from a MAG article
http://thames-valley-region.mag-uk.org/Pages/exhaust_legal.htm
The new MOT legislation requires that the vehicle tester is able to see the standard (original makers stamp) or aftermarket exhaust fitted to the bike is marked with the obligitory BSAU 193a and the European "E" mark. If your Exhaust can does not carry these markings, it could lead to a failed MOT and if you are stopped by the Police it can lead to a fine and points on your licence.
That was lifted from the MAg article, the noise test is up to the tester but the exhaust needs those markings, the tester can fail it if it doesn't have them and even if it does if, in his opinion, its too loud. The reason for this last bit is obvious - cored cans and removable baffle systems which might be marked but emit more noise than acceptable.
if you want chapter and verse try reading the government guidelines and advice to police officers on exhaust marking then see if you want to argue in a court of law.
Interim report of the Advisory Group on Motorcycling
and the bit from DOT where they clarify the requirement (and also indicate its complex and more regs exist) but also the last bit which I'd never seen before and might be worth investigation
(d) Exhausts - on motorcycles first used on or after 1 January 1985 the silencer which forms part of the exhaust system must be either:
(i) that with which the machine was first fitted; or
(ii) clearly and indelibly marked with :-
-the relevant BS marking BS AU 193/T2, BS AU 193a, 1990/T2, BS AU 193a, 1990/T3; or
-the relevant "e" marking to show compliance with EC Directive 89/235; or the relevant "e" marking to show compliance with Chapter 9 of EC Directive 97/24; or
-the name or trade mark of the manufacturer or marked with that manufacturer's part number - relating to it.
The European Community noise limits applicable to new motorcycles first used from 1 April 1991 are:
Motorcycle Category by cm3
Limits in dB(A)
Up to and including 80 = 77
Between 80 and 175 (incl.) = 79
Above 175 = 82
Motorcycles approved to EU Directive 97/24/EC, Chapter 9 will be 2dB(A) less than the above figures.
(Note: the precise regulations for motorcycle exhausts are complex and it is recommended that they are studied closely to obtain accurate and complete details of the requirements).
Before buying any replacement parts for systems listed in this section, riders should check for the relevant marks, where applicable. These will include an "e" mark for EC Directives, an "E" mark for ECE Regulations and "BS" for British Standards.
(Note: amateur motorcycle builders may be exempt from some or all of the requirements)
Its lifted from
this document on powered 2 wheel vehicle regulations.