Just to update if anyone interested in the follow up - I mentioned the fit of the piston head being loose - thanks to ggggr for the tip to use a thin piece of leather. I cut a piece ever so slightly oversized from thin leather (taken from Poundshop riggers gloves - always a useful source for small leather items, especially for catapult pouches ) and after soaking overnight together with the original cup washer I test fitted it all today to try in the cylinder.

A lot of fine trimming but spot on now and a very snug fit. Not sure if it benefits in adding a slight second seal but with finger over the port there is a gentle air spring effect now and the fit in the cylinder is beautifully snug but smooth too.