Found the handbook. Not too bad really. Does at least tell me how to get the bottom cover off to see inside the "works" and what all the bits are inside. Also tells me
a. the battery is CR2032 as Zinglebarb indicated.
b. That checksum errors usually mean the battery needs replacing.
c. This is a fairly easy "user" operation
Tomorrow I will be dragged by the Mrs to climb some damnd hill in Wales so it looks like Monday before I can sort this one out
I seem to remember an old expression "when all else fails RTFM".
