Ian Kelly had a link to a history ( which he had written ) of what was basically the Territorial Royal Signals units of the the North West on his website. From memory the history contained a number of lists of officers who had served at various times in various units. The link no longer appears to be on his website and the only mention I can find of the history is here:
http://archive.is/ZOPuq
Unfortunately I do not appear to get beyond the various headings in the link and whilst I was hoping to check if Captain/ Major Johnstone was mentioned in the history, in the circumstances I cannot.
I bump into Ian Kelly regularly and if I can remember I will ask him if the name means anything to him.
P.B.