Yep Steve- I concur with your Parents.
My lot would remove the plate and I could hear my younger Brother coming from miles away at night (on the odd occasion I was back from the Pub before him) - they had that distinctive roar- the 4 cyclinders helped to make it smooth- In them there days we were used to Brit Bikes- twins- making a din- so these were a nice change! He took me to work early one Saturday up Regent St- I used to work at the BBC in those days- the noise was deafening, as it bounced of the glazed shop-fronts. I think I had one on a 400/4 I had in the late 80's, but i cant remember..so many bikes- but Dunstall was local to us in s/e London and went hand-in-hand with a 400/4.
I know if my wife heard it (should i fit one)- she would say 'DUNSTALL' as its a raor you dont forget!