the other half (who is right clever when it comes to blowing stuff up) says that batteries subjected to temperature fluctuations become very volatile, the temperature change does not have to be great, just enough to start the chemicals turning into gases, then cooling and then slight heating again, do it enough times and you have a tiny hand grenade. its reasonable to assume you have you turned the central heating on in last few weeks and that this has started the chain of events. :-)