Thanks for the responses, on the basis of which I'll hang on.
I was planning to cascade this to a daughter so I will probably have a new battery installed at that point, having transferred everything to that spanking new MBP 😉
The number of cycles reported before replacement seems to be highly variable though - noted the comment about the deterioration if always docked. My usage involves frequent powering up overnight but then mostly relying on the battery, topping up as required.
btw, I had the screen replaced when there was a call-in on that and I've upgraded the SSD to 1gb, so if the battery replacement means a new keyboard then my cascade will be a bit like "my grandfather's axe" 😉