Thanks guys, after an hour on the phone to Telecom - who didn't tell me to go to their Thunderbird help page - probably didn't know about it - but I will now, they had to admit defeat and were unable to offer any suggestions, other than not to go to IMAP - as Telecom NZ don't support IMAP, they said !
I then uninstalled Thunderbird and re-downloaded it, and the problem has gone away - for now.
I still suspect that Telecom had a hand in it, tho' they denied it and said it must be a Thunderbird application problem that they couldn't help with.