Was it a submarine bell?
I contacted the Royal Navy Submarine Museum Curator and sent him photos of the bell and asked his opinion about the possibility of the bell being from a submarine. I received the following reply:
All the bells in our collection are from commissioned British submarines and thus the bell always bears the name or number of the boat. The lack of this on your bell doesn't make sense if its got anything to do with the Royal Navy and submarines. Its also quite big and the shape is not like any of our bells. So if I had to put money on it I'd say no, not a submarine bell.
So there you have it! Always check provenance! I am still delighted with my bell. It is a fine bronze bell obviously used in an important building for some kind of warning. I shall probably never know. Unless............. someone, somewhere, reads my blog and enlightens me!?