Thanks for the replies.
Had the A1200 powered down for three and a half days then powered up and the clock has reset itself to 01.54, 20 June 1996, which is the date the Workbench partition was written to disk.
I'll buy a new battery today and see if that cures the problem. The current battery has only been in there about a year though and I'm told they should last about two years (CR2032 type battery). I suppose it could be a duff one.
Nemo