Thanks for the responses, lots of useful info there.
The battery wasn't performing poorly I think it's quite a common feature of Li-Ion batteries that they give a very stable output right till the end of their charge at which point they drop off quite quickly. The camera measures that drop to determine when it's running low so it's warning that you are running out of batter doesn't tend to come till just before it's fit to die. That said I did find the article you referenced, for anyone else wanting to check their batteries here is a link to it on the UK version of their site.
It turns out my battery is one of those that has been recalled. Chance of an issue is very low & only while on charge, however if Nikon are willing to give me a free brand new battery in exchange for my old one who am I to turn them down. I think my battery has only been through about 6 charge cycles so it still manages about 1500 exposures per charge, but it did sit on a shelf for for quite a few years and there is apparently a very slim chance that it could explode so better safe than sorry.
I phoned up Nikon to ask what I need to do and they have emailed me all the forms I need to fill out this morning. I told them I've got another battery on the way and so could I hold fire on returning this one till I have the spare so I don't have any down time and they were fine with that. I actually got the impression that if I wanted to leave it another 6 months they would be fine with that too.