STOP! Hi Gerard---as soon as I got off the computer last night I thought. I take it your fulcrum plate is not worn? Ok,take the barrel off and take the mainspring out before you look at the trigger again. Try to cock the gun by moving the piston with a screwdriver/small fod etc. If it now cocks and also holds while you push the piston, then your trigger set up is probably all right. If it seems to be, then try doing it with the barrel back on but still with no mainspring. If it does not cock now or seems marginal, look at the little cocking link Bk118 https://www.gunspares.co.uk/products...est-1979-2005/
It may be worn flat at the front and cannot pull the piston far enough back. You can knock out the pin and reverse it until you get a spare one.
If that does not work then it is into the trigger again.