Got it. Thinking further, I realised I hadn't made any changes, so it must be something external, and the time stamp was my immediate suspect.
I've been using the SetupBuilder 10 drop-down option
http://timestamp.comodoca.com/authenticode
and (having seen a recommendation to use comcoda in a previous thread) I'd tried the alternative
http://timestamp.comodoca.com/rfc3161
but without success. However I now find that switching to
http://timestamp.digicert.com
works!
Wish I knew what was causing the problem!
Dave