You have come up with quite a few ideas Rog. Thank you. The most likely would be 1901 Edward VII coronation., because I think they look about 20'ish especially great nan.
They are definately younger than 30's. I have another photo of them with some of their children which must have been taken at least 10 years after this photo, and they do look 30ish.
Thought it might also be possible that it could be for Queen Victoria's Diamond jubilee in 1897! What do you think?
Sue