I did a quick stripdown and the schrader valve broke but l managed to get it out.
l recieved all my rebuild parts from Chambers but l decided to just replace the valve with an automotive valve for a bit of practice. Everything inside looked clean and the inertia mass appeared to move easily.
So l decided to reassemble everything and pump it up to approximately 20 bar.
So far so good as it's held pressure for over 36 hours and its shooting very nicely and very accurately.
l am very impressed by the simplicity (and quality) of it all.
Thanks for the tip about the Schrader valve torque driver as l didn't know there was such a tool - when l tightened the valve l was trying to not overdo it.