Update - as advised by Lovel above, went round to Phil Hemsley's last night and got the a/c recharged. Lovel - did you have dodgy a/c? When I told him what kind of car on the phone he said "Oh, yeah, did you have problems with that before?"
£40 all in, plus he gave me a couple of cable ties to hold my undertray in place till I got home (must've vibrated the rear fasteners off on the way down his track.
For comparison, the local Renault dealer in Memsie (only other place round here that does a/c just now) wanted £90 + VAT for a recharge. I made my excuses and left.
Angus