I remember reading about them in a history book: I think they were big before the 40s, (especially WW1 where they supplied the troops and thus I think came the name 'marching chocolate') but suffered badly during WWII when the factory got damaged and a lot of chocolate production ceased anyway. They were taken over after that (possibly by Mackintosh (?), which would probably mean they got dragged into the whole Rowntree-then-Nestle-etc. fiasco) and kinda phased out of existence but seem to have re-appeared recently in a nostalgia type thing.
I had some cocoa of their make a few years ago, nothing special from what I remember, but I have seen their Marching chocolate in a lovely traditional confectioners in Cirencester and always thought it looked lovely and nostalgic, and reminded me of my grandparents (though I know its all really just a shallow and cleverly constructed marketing ploy).
Apologies for the essay,I don't know if it helps. Your shop sounds very interesting and and individualistic! I always love discovering random, especially small, traditional and homegrown types of chocolate manufacturers.