Henk are you sure it's in Arnhem? If so, it must be left behind by the Canadians, as far as I know the airbornes didn't use any BSA's during the battle. Also I've never heard of this story and I am from Arnhem! Maybe my friends are just not telling me everything as usual.
Based on the camo, it is probably Belgium. And with the two photographs published, one in the October 1944 Motor Cycle and one in Motor Cycling showing two BSA WM20's outflanking a WD-G3L Matchless inside a Horsa, I would assume that there were BSA's at Arnhem. It is like the Welbike story probably: "I have seen no pictures, so they can't have been there!"-crap?
looks like left over bits chucked in a hole, the bike was missing big lumps of iron, barrels etc, which dont disappear by themselves, probably its a load of left over bits from a workshop chucked in a hole somewhere, which is not an uncommon practice.
kit what do you mean the dutch are getting a bit keen they have always been keen. on the dug up bike there didnt seem to be any pannier carriers early model m20 and it would be nice if we knew the frame number