There has been a change in the MOT regs for exhausts from June 2004 , The new regs are simpler on paper but also open to more interpretation
All Systems The Exhaust system should be in good condition (no rust holes / complete and secure ) and be correctly fitted (ie: not blowing from the joints ) durable repairs are acceptable providing the exhaust is structually sound.
There should be no leaks which cause excessive noise
The next part I will quote direct from the testers manual
Section 7.1 reason for rejection 2
" A silencer that 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 "
IF the EXHAUST is marked NOT FOR ROAD USE or TRACK USE ONLY or similar words it should fail an MOT.