You could research a bit more to find more about your WW1 cover, Whizard.
This one of mine is a year earlier than yours (1916) and I can tell it was posted at what was called
The Western Front as it has a hexagonal censor mark.In January 1916 triangular marks on the Western Front only were replaced by hexagonal marks. The Army Post Offices were given numbers to identify where they were... but I believe they changed numbers more often when it was realised you could rebuild whole battalion locations if mail was captured.

There were 7 types of censor marks...Circular, Square, Triangular, Hexagonal,Oval, Rectangular and Shield. Interesting subject to research should you wish.