I called Apple and after being cut off (grrr) they agreed to send me a new battery free of charge. Apparently this one was subject to a product recall anyway. The thing that annoys me is that they knew I had one of these batteries, they have my email, telephone number and address and could easily have informed me but didn't bother. I think that's a bit naughty really.
Once I got through to AppleCare they were very quick to ask all kinds of legal questions, damage to house, person, computer etc but wouldn't have needed to worry whether I died in a blazing fire if they'd have informed me that the battery I was using was potentially dangerous.
Anyway, all sorted, thanks for the advice guys.