I don't buy it myself. If I had the time I would investigate further just for my own interest.
The ruling on atmo bovs is because of emissions thbomk. The noise thing on a plumb back is a crock of shit imo, I have yet to see a dB figure that a bov must comply to, and as much as main roads want to think their job is subjective opinion, it's not. You comply to the rule, or you don't.
Maybe I'm just getting too old to put up with the bullshit nowadays........... B reg all the way!!!