mate not got back to me yet.
ps got this info off scooter
Arnout said "1873 I think" when i asked him. So you basically need a belt with the reference 6PK1873.
The little book you get in Halfords has lists of similar belts 6PK1953...1763 etc etc. If i need to do it i'm going to go for a '1873' or slightly smaller.
Unfortunately i didn't find an exact match in the halfords book (but i didn't look long just at some possibles - ie larger v6 engined saabs vauxhalls etc)
However the belts are available in 5mm intervals over the range we're looking at so i suspect it could be specially ordered by halfords, or another good (better) motorfactor.