Thanks to all for your continued sleuthing.
I've gone back with my magnifying glass, and the poster in the background definitely says "Cheltenham Coliseum", and on the frame it says "K.L. Beard's Theatres" (or possibly M.L. ...). The other thing that struck me is that everything about the building looks brand new, which could explain why it isn't in Koromo's 1913 trade list, the photo being from 1923. In fact, perhaps this was the opening night for the theatre, which would justify a big group photo.
And great information from Doug ... I'm sure your distant relatives and mine knew each other, since mine were an extended Yorkshire-based theatrical family.
Cheers
Tim