It's really not that bad, Tone.
The more Pistons I weigh out of my spares pile, the more I'm convinced that the D35's piston weight is an issue.
Diana 52 piston = 300 grams.
Webley Vulcan piston = 270grams.
Pre war D35 piston = 250grams
MK2 Airsporter piston = 307grams.
So with the D35's piston weighing in at @ 240grams I reckon I could start off by adding at least 40-50grams in weight to it then use a lot less Spring than it came with.
Anyway I'll carry on thinking, I may be best to just rip my 11+ftlbs Mercury apart and replicate the internal dimension in the D35 --- it would save a lot of messing about.
All the best Mick