Quick update RE: GM120 - It runs & holds gas well! (better than my Telcos anyway!) So it does appear that the shape of the valve is not critical...good news
@ Squirrelnet - I was pretty taken by an article I read about the CO2 scaled 'Bantam' that Dave Deadman had reduced from a Ron Warring design. I have the plan & it is a beautiful design, so I'm very tempted to go with that (its reputed to be a fantastic performer). I've also got an Aerographics Gladiator for GM300 and a Veron Dart Kitten for Telco... and very little building time (house move & young kids).
All projects for the (hopefully not too distant) future.