The watch in question (CASIO 1572)

This was harder than expected and I did not find any instructions on the internet – the manual from the CASIO website was useless.

The battery required was a CR 2016 which I got from maplin.

Getting the battery out and putting a new one in was not too hard – taking the back off required a small philips screwdriver and then the central section was levered out and the inner metal cover removed by unclipping it. Putting the new battery in was also similarly easy.

However this did not make the watch work – it was still blank and needed reseting – though the light worked. I could find no way of fixing it and so in the end I held down the light button and poked the copper contacts inside with a pin connecting them to the outer metal case in the hope of flipping some switch somewhere and so making it work – to my surprise this worked.

Note: that there is a little spring inside the case and so be very careful when turning the inside section upside down to keep your finger over the spring to stop it falling out.