Thanks for the thoughts. No, definitely not related to the magazines. All are genuine Weihrauch ones and seem to be working correctly and feeding well.I have tried several pellets, From Rifle and Pistol weight match grade R10s, which are my own favourite pellets. RWS Super Fields, JSB exacts, Bisley Practice, Bisley Magnums, H&N FTTs and so on. They all give the same random results.
My chrono does not show up any glaring errors for energy or fps. Had another look at the barrel this afternoon, both inside and out, and at the rifle mounting points. Removed the cylinder and checked every visible seal and mating surface. Everything looked to be in the expected order. Re charging the cylinder was easy and I did it very slowly to try and detect anything untoward or leaking as it reached its working pressure.
The grub screw holding the barrel appeared to be utterly secure. there was no lateral movement or axial end float in the barrel when mounted. The crown looked untouched. The bore looked untouched by anything sinister. The visible working parts after the stock was removed looked to be working correctly. The magazines indexed correctly. At one point I thought that the barrel was laying somewhat loosely in the stock very slightly, noticeable by a very slight rattle when the gun was moved laterally. The two securing screws were nipped up a little tighter and the rattle was gone but the random POI remained.
I tried to see if the gun would behave any differently with a different telescopic sight fitted. It made no difference to the POi. The expected POI was not what was actually achieved in test firing 70 plus pellets. Nothing really helped the gun to show the groups that it was capable of for the last 18 months. I am at a loss to know what to do next.