Yes, it's a Li-ION and it's brand new. I think I've found the problem - it seems to be the battery charger itself. I ran the battery down until the alarm sounded then left it on the BIOS until the battery ran out completely. After restarting on mains, the computer wouldn't charge the battery at all until I took it out/reinserted several times, after which the power meter still read 83%! Something similar happened with the old battery too, if I recall correctly. Unfortunately my BIOS is very basic and there are no facilities for recalibration.
As I use the laptop as a desktop it's no big deal but if I ever need to carry it around I'll need to look at a new one, having already wasted 100 quid on a new battery
Better start buying lottery tickets again.
Anyway thanks for the suggestions, Stu - nice to know there are people out there who are prepared to take the time to help.
Neil