An update.
TNA replied "Our initial examination of your [copying] request shows that specific information within this document is not easily identifiable and will require further research." and they then give the usual research options.
My first reaction was one of disappointment, that the whole file could not be copied. However the implication is that this is a big file which, from my point of view, is a good thing. Walter McFarlane had a very long military career and the more information the better.
I know a good researcher in London who specialises in military. I will get him to look at the file and also the Fort George muster rolls.
In addition to what have must been a fascinating military career, I am hoping it will also give me a lead on Walter's family from Arrochar/Loch Lomond.
Bob