Did a bit more, bit off a faff cleaning out the bottom of the cylinder, used various bits and bobs Emery glued to the end of a delrin plug and soaked it in acetone, but finally I have a nice clean face , o ring and piston ringed pistons like a nice square face to land on. My end block has a slight concave bit in the middle where the machining hadn't cleaned up so some lost volume there.
I made up the parts for my piston today and glued them up ready for final machining.
https://photos.app.goo.gl/R4kff58TcxLGsXCV8
https://photos.app.goo.gl/J8objJYGBdUDUffJ6
Will cut a groove for a standard sized o ring and then I have the option to use either that or piston ring
Oh and machined a flat on the cocking shoe to run against the rear of the piston.