Test fires showed it was producing 9.5lb.
After talking to a very knowledgeable guy Jon Budd he suggested i lightened the piston, which i have done. Three slots have been cut out down its length, and i have also replaced the spring guide and piston weight with a delrin one i knocked up on the lathe.
The piston weighed in at a tad short of 400 g

I will put it all back together in the next few days and will share my results.
BBF