Thanks for the welcome back.
So here is where I am at the moment.
As recommended many moons ago I have added a large washer to secure the rubber face seal to the front of the piston. As before this is attached with an M10 screw.
Pressed into the rubber face seal is a sleeve to allow the screw to clamp the seal tight and not over compress the rubber
I have also moved the O-Ring grooves further apart.
Here is an exploded view for clarity
I plan on performing some more stress analysis on the piston before cutting metal, although I'm not entirely sure how to setup the analysis within SolidWorks (in particular what fixture face I should be using) - if anyone has any guidance that would be greatly appreciated.
Regards
Stuart