My first thought is that it is due to oil/grease residue inside the piston/spring bore - see if you can clean the bore out at all mate? You could get someone who is confident to do a strip down, clean and re-assembly i guess, or if you are confident enough you could do it yourself?
Failing all else, take it to a gun shop - i am not sure what the charge will be, mind you.
Regards,
Grim.
Grim,
The rifle has stood for 15 years and only had a few plinks before that. I'll bang some through and see what happens. May strip and see what's inside if it gets no better. I'll see if the