Many thanks. Lots of ideas here. It could be some crud in the breech area which causes a one-off problem but moves along. Shooting in dusty, muddy woods isn't the same as indoor 10m/25m :-).
I have a habit of opening the breech before getting up and sometimes use a breech flag (bit of a safety habit from smallbore). With the Anschutz the block slides back quite a ways and exposes it to crud but I just tried an elastic band to hold it against the barrel but not leave it compressing the seal which may be a better way of transporting it to and from shoots. The Walther is better in that with the breech relaxed the push rod still sits in the barrel end.
I don't think I could load a pellet backwards in an Anshcutz if I tried but it is very easy on a Walter with the more cuboid 7.9gr (Express or Mosquito) as they tend to roll tail first in the loading groove. Less of an issue with Exacts which are longer.
Sorry Simon - I have tried weighing and shooting groups in the past and the answer is a better batch :-) When I did group them by weight and then measured them I found the slightly heavier were slightly longer than the median, same for lighter (shorter) so that is likely small variations in the wire lengths for JSBs.
I throw out the slightly deformed ones (and even plinking with them they go straight enough at sub-45yds).
Besides, they can't be bad pellets as I buy them from Intershoot and the N.I. folks test the batches hahaha
It may very well be a nasty twitch on my part but I think it more likely pellet/breech. Hammer issues would be tedious and involve more dismantling than I care to do unless the flyers increase a lot more :-)
Many thanks all