Oh wow that is one blinged up gun! I wound up doing a complete strip down and removed the Co2 cart still attached to the valve ! I then had to put the Co2 cart into the vice and knock the valve off with a spanner/mallet combination not nice. On reassembly I found the transfer port seal loose in the gas chamber and fitted it correctly this may be the poor performance cause as it has been poor from new so I believe the quality control at assembly was at fault. Tired now but will run a string tomorrow and see what that brings.
Thanks very much for your input, drilling the side of the air chamber seems like an idea if this becomes a persistent problem.
Many thanks for your help.
Mick