Thanks for all the suggestions on how to differentiate officers and NCOs.
While the grey brick worked on the painting table it was next to useless on the board! So, I returned to what to me is aesthetically less pleasing but actually makes the game playable without picking up every German mini to see who it is...... Yep, as also suggested by Matt, I went for a clump of weeds.
I used the dead grass GW Mordheim clumps, which needed a spot of superglue to fix them to the varnish.
It's now a lot easier to spot the NCO at the centre of this German attack column:

The British ranks were always easier to pick out but for consistency I did them too.

I'm pondering trimming the strands a little shorter .....
EDIT:
I trimmed them considerably shorter, which is much more pleasing to the eye (though makes them a little less visible. They do need a touch more Matte to cover stray flecks of superglue).