Tony, at least replacing the CMOS battery is a quick and cheap step - if it works, great, if not you've only spent a couple of quid.
I'm in the process of doing just that with my laptop - the time is back to 1980 so it's definitely a dead CMOS battery. It also has a problem with the main battery no longer charging at all. But first off, I'm going to see if changing the CMOS battery will get it to charge the main battery. I'm not hopeful, and expect to have to shell out £40 or so on a replacement, but you never know.
SD