To answer an old query:
I really love this song and perform it a lot. The older generation in the audience usually join in. But (sorry to go on about this ) every time we sing it in public, do we have to tell the venue and tell them to pay PRS?

Apparently, YES.
According to Wikipaedia (that estimable & 100% accurate resource, cough cough) Fred Fisher died January 1942, but his daughter who is also credited with the song died Jan 2003.