Latest ! I've telephoned Daystate .( They like you to phone after 4-00p.m. with technical queries ) and a very nice lady explained that :-
a, there may not be a fast enough exhaust of air when I press the relief valve to close off the non-return valve or
b, there is some dust or grit in the valve..
She explained that if the gun is empty of air I can remove the male filler fitting and clean out the valve with a small brush etc.
Guy at BAR said to try and open the cylinder valve really fast when filling in order to try and blast any grit or dust out of the valve. This did not work but I found that my cylinder is now low after 9 or 10 wasted fillings so I took it for fill and the cert has just expired ! Its going to be a week now before I can play with things again and sight in the Ziess I was hoping to use on the rifle.

Oh dear! 'Ow sad , what a pity , never mind !