O.K. folks, sorry to waste you time.
Apologies to the good ship Slack and all who sail in her.
Laptop was powered down all night.
Doesn't show the symptoms today. Have tried various time zones, and they all work as expected (ntpdate commented out in rc.local).
We actually have a time zone that is half an hour behind the zone I'm in. Looked like that was being used incorrectly.
NO - I didn't use this other zone at any time yesterday. Deliberately didn't, as things looked so suspicious.
Dunno what caused it, unable to reproduce it. ...