Steve, probably sent along inside a letter during the winter of 1939 / '40 with no information due to censorship regulations. The snow came early in 1939, and as the movement serial hasn't been overpainted, I'd imagine that this was late 1939. They arrived Cherbourg 6/11/39 and were allocated immediately to GHQ Arras. It must be somewhere in that area.
The lattice structures are for power cables I think, see attached link to BEF photos, go down to the Bren carrier picture at Orchies and there are more in the background.