In the interim I have:
1. regenerated my Pst file
2. Repaired the entire Office suite
3. Created a new IMAP e-mail account and tested it
4. tested sending to and replying from the account from a web-based mail program.
No change.
Latest suggestion from somewhere is to try this:
http://www.msoutlook.info/question/c...ows-10-upgrade.
I'll keep you posted.