Thanks majm. While I wouldn't rule France out I'm now fairly convinced that tonepad was on the right track from the very beginning, suggesting Pembroke (
reply 2) or Glamorgan (
reply 6) Yeomanry in a typically British landscape suggestive of a summer training camp before WW1 started in Aug 1914 (
reply 17).
I think Occam's razor points to that answer too.
My personal odds-on favourite (but still not definite) is now the tented Yeomanry camp at Penally (near Tenby, Glamorganshire) before or in the early stages of WWI.