Hi M BLUR (really senior member).
Thanks so much for your helpful and most intuative comments (I'd never thought of 'body kits').
Unfortunately, however, you've missed the point as 20" diameter (18" or even 22") makes no difference (providing you stick to the same 'rolling diameter') and is therefore NOT my problem.
Wheel WIDTH (in relation to the face of the wheel arch) is my point (4.4 wheels are 285mm wide, 4.8 wheels at the rear are 315mm wide) and that extra 30mm brings my wheels right out flush with the body at the top of the arch (that's exactly why the wheel arch trim on the 4.8 is flared out; to COPE with the extra wheel width).
Regards - S24PAW
NB : At £2500 plus say £80/hour for fitting 'bodykits', I'm naturally trying to find myself a more inexpensive solution.