Some good news to report folks.
Today I switched over the piston head to a long stroked O.E one and put a quad ring seal in. I also triple checked the cocking shoe which was clear even with the Welsh Willy head. It is now further from the piston.
The new buffer washer had to be sanded slightly to get it in and reassembled her.

I fired a few Marksmans and tested velocity 508 fps or 11 fpe. More importantly there is now a nice soft plop and no clatter/slam noise.

Wanting to check for dieseling I fired another 30 or so and checked velocity again

520 fps or 11.5 fpe. I am now a happy bunny.

This time I peered down the cylinder with a torch and could see a slight ring of metal sticking out around the cylinder end.
A parachute seal would have hit this but the BSA piston head would fit inside with the O ring seal just behind.
Clearly what I thought to be a tight choke has made no difference.

Power without control is however, no use to any one so now I've got to fit a scope and the shim washers which as the breech block is smaller than a 99 may be visible, then test for accuracy.