I didn't get to change the battery as I wouldn't part with £67 for a replacement Graham! :)
The battery is indeed unusual, in that it has four lugs moulded to it as Jeff suggested. Not sure how anyone could replace it with a non-B&O substitute as it fits tightly into one side of the BS2 casing.
I know my limits and doubt I'd ever open it again! To check everything works, I plugged it into the docking station last night to charge and upload some files and it seemed fine. It was also fine to listen to this morning, apart from the fact that pressing the down button did not take me back to the start of the current song, but rather to the previous track in that playlist. I can't remember if it always did this, but if that's the extent of the functionality I have lost then I'm a happy man.
I did wonder whether this small resistor (or whatever it is - there are a few of them mounted on the PCB) had dropped off my board, or had always been floating around inside the case as I would have expected it to stop working after my ham-fisted attempts to open it up...?