Steve, not debating which, why, who or what.
My original post gives a link in which a car owner with an after-market exhaust, is questioning an MOT failure.
I know it is very prominent within the 2 wheeled world, however it seems it is applicable in the 4 wheeled world, it is entirely subjective, one MOT tester may fail a car that another passes.
Taken from the Direct Gov website.
Exhaust system - complete, secure, without serious leaks and silences effectively
Taken from the MOT testers handbook (note the formatting is up the spout, but you get the idea)
1. Examine the condition of the whole exhaust system, including the silencers and mountings, for security, deterioration and completeness
Note: The overall security of the exhaust system should be assessed; one or more exhaust mountings missing may not be a Reason for Rejection.
2. With the engine running
a. check the exhaust system for leaks
b. assess subjectively the effectiveness of the silencer in reducing exhaust noise to a level considered to be average for the vehicle.
Note: The level of exhaust noise should be assesed at all engine speeds required for the exhaust emissions check.
1.
a. A part of the system missing or excessively deteriorated By QD- Non Res?
b. an exhaust system not adequately supported.
2.
a. A major leak of exhaust gases from any part of the system
Note: A minor exhaust leak from, for example, a connection joint or a pin hole, is not a Reason for Rejection
b. a silencer in such condition, or of such a type, that the noise emitted from the vehicle is clearly unreasonably above the level expected from a similar vehicle with a silencer in average condition.
If it fails, take it to another test station. Pays your money and take s your choice, I personally think along with other aspects is a sign of things to come.