Stephen,
Thanks again for the advice. I will write to them directly. It is interesting as some more evidence from JT's excellent collection points towards such a change. This
picture shows 2 Lt. Cyril Thomas Underhay As an 11th Bn. Officer (according to his collar badge). However, his MiC also states 21st London, which would mean he seems to be wearing a yellow club patch for that battallion for 1918.
It would be nice to get the definitive answer for my query, so I will take your advice and drop them a line.
Cheers,
Chris