Thanks chaps. The spring appears to be quite soft 2.8mm wire according to my middle of Lidl vernier gauge, 33 coils long. Crossing threads I have used a nylon slip washer and spare power washer totaling 6.4 mm of preload.
It did boing badly so I've put a Guinness can piston sleeve in to make close sliding fit and gently polished the bearing end of the piston with 1500 grade wet & dry. The spring ends have come up nicely with file & 1000g.
I'm still to polish the cylinder with 1000 grade and report the piston weight when the scales arrive.
I'm quite impressed with the spring guide, metal with a stepped plastic collar/sleeve which would allow rotation of the spring rear end.
I will test for leaks upon reassembly and look up a Titan in the meantime.