xr craig, not saying you have.... How many of us have probably p*ssed off an off duty scuffer? Overtaken one at full chat, drowned out his 'Jazz Fm' with some humming bass lines? Well, some of us, ahem! He's got your card marked for one reason or another. If all he could winge about was your exhaust i wouldn't worry too much.
There is a Ministry Of Transport web site, which will have the applicable figures you want. Try Maplins, or the like for a Sound Pressure Level meter about £20. That way you can test your car, if you fall under the SPL level, carry the meter in your car, if he scuffs you again, you can perform the test for him. Can't say fairer than that ( what a friend of mine used to do with his 100E ).
British Kite mark, hmmm, so what if you had a custom silencer made for your car? Me thinks he's clutching at straws on that one .