After owning the N900 for a few weeks and watching the battery life drain away quite quickly I went and bought a new battery from Amazon. I'm planning on using eCoach for long bike rides so I new 3G/GPS would be on a long while and that eats up the battery.
So I charged up my new purchase, and its much much better life than the battery that came with the phone. 1 hour bike ride and virtually no life used up on the battery - compared with 30-40% life with the origial battery.

Wonder if I just got a bad battery with the phone originally, but I read lots of folk having battery troubles - maybe a £5 at amazon is a good investment.