Thanks for that, Maris. Wonderful picture, with an amazing amount of detail.
The frustrating thing is that the background is of course not in focus. From what one can see, the roof structure looks unusual, with a lot of cross bracing and in one area almost a latticework. The signs to the left remain tantalising and they certainly look like railway signs. And in what other environment would one have large signs like that either within or just outside a large building, other than in a railway station? I wonder if anybody knows what the roofs of Snow Hill and New Street stations were like at that time.
I still think it is a group of catering staff, perhaps with others in the picture. The waitresses are all numbered, presumably allocated to particular tables. The chef, back row second left, has a disgracefully dirty "white" coat on; but on the other hand everybody else is spick and span. I like the lady on the extreme right who obviously prefers to be photographed in profile and otherwise treats the camera with disdain. As you say the waitresses seem to have the letter "R" on their caps, presumably the name of the company or hotel which employ them.
All one can say for reasonably certain is that the picture was taken in the spring. (Daffodils on the table - they won't be plastic!).
As for your grandmother, far be it from me to try to assess the age of a lady. But (if I have identified her correctly) I do feel that she could fall within quite a wide age range and I suppose that the only way of establishing it is for you to compare the image with photographs of her from a known date.
How nice it would be to establish when, where and why.
Chris