As per the MOT Testers' Manual (information taken from
here), reasons for failing an MOT on the "exhaust system" are:
1.
a. A part of the system missing or excessively deteriorated
b. an exhaust system mounting missing, or In such condition that it does not fully support the exhaust system.
Note: An alternative mounting device is acceptable providing the system is secure and the fixing is not likely to fail prematurely-
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