After some experimenting I've revised the basing technique and started using Vallejo Buff 976 as the colour onto which the brown flock is tipped rather than the Revell Khakibraun 86 I was using before. This looks a million times better, the castings "pop" out and the texture looks three-dimensional rather than flat. Water-logged ground remains a mix of blue and grey with white blending highlights/reflections. I'll be using this from now on and might re-base the dozen or British bases I'd already done in the darker form.
Three companies of Tommy Aitkens with machine guns and a piece of Low Countries cathedral. The water-logged ground will be gloss varnished after the bases are sprayed with Citadel Purity Seal.
I've already got a nice, "green" basing technique sorted out that was used on my Thirty Years War pike and shot figures but wanted to avoid any grass on these bases, being as they were intended for the period from 1915 to the Kaiserschlacht and Hundred Days Offensive. Grass flocking would have lightened the bases and brought the models out but was thematically wrong.
So what happens next? I'm in Waterstones, I see the Osprey Campaign for the allied counter-attack at Amiens in 1918, known as "The Black Day of the German Army" buy that, get home, put the kettle on, sit down with a cup of Assam and the book and lo and behold! All the pics of the Amiens offensive show troops fighting and marching in nice pastoral fields and villages and intact woodland all a long way from the shattered Golgotha of No Man's Land and very reminiscent of the landscapes of summer 1914. Oh well.