....Everything I know (which some may say is very limited!!) tells me that
ANY reduction/obstruction of an air intake is to be seriously avoided! The idea is to
INCREASE air flow/intake, not to
REDUCE it!!

It'll be interesting to hear the outcome of your number plate location enquiries, Hurdy. But I doubt you'll get anything in writing and without an official piece of paper, how are you going to insist you are legal when Mr Policeman stops you and you start arguing your case? I hope I'm wrong, but....