It is part of the MOT I believe..
I think it's at the testers discretion.
When I first got my green 420, it had a completely standard exhaust system bar a backbox with a 4" outlet. It was so loud it made the viewing booth vibrate when doing the fast idle test

Got a few comments but made it through the test.
Oh and before anyone says anything... it was eventually changed for a quieter but better system
