Thanks to those of you that suggested I used constant agitation when using the S toekler formula to develop my B&W sheet and rollfilm - this certainly got rid of the "streamers" nr the edges of the film.
However, one thing remains as a constant - banding in large areas of even densit y like skies. Not every sheet is affected, not even every frame in a strip of r oll film is affected, but I cant stop this phenomenon from happening.
When it occurs (and it occurs with annoying regularity) it looks like banding af fecting skies BUT it seems to stop a little way into the "busier" areas of the f ilm. Most of the negs that are affected also show some tonal bleed into the fil m rebate adjacent to the banded areas. In fact, on roll film the sky part of th e neg looks like it has an echo/outline, as if there has been a double exposure (but I know that there hasn't).
I've tried different film/dev combos (eg Agfapan 100, Delta 100, ID11 at 1+1 and 1+3) but still the banding appears. I've tried agitating drums by hand: 10secs at the head of each minute, 5 secs every 30 secs, and also a unicolor drum on a reversing base, to no avail.