Whilst the answer is not actually in the Forum, the inspiration came from several of the articles. I did all the usual, re-installing phone software with Nokia Updater (which does not work on Vista at the moment – did it on a Windows 2000 machine), removing the battery overnight, tearing my hair out, and was going to do the hard reset mentioned in some postings (*#7370#) but decided to sleep on it, and woke up with a Eureka moment.
I backed up everything, except calendar which Nokia PC Suite could not do, but as I sync with Outlook anyway, not a problem.
Changed the year back one year to 2007 so that phone looking at year old appointments and et voila is if by magic, calendar back and working.
On Microsoft Outlook Looked at the appointments in the date range that I was previously synchronising, but found nothing untoward.
Reset date to today
Set Nokia PC Suite to synchronise, first a couple of days, then weeks, then months and finally a year and all synchronised OK.
So no idea what caused the glitch or why it subsequently OK, but can only assume that there was some conflict during one synchronisation that caused it all.
Anyway Calendar up and running and synchronising again. Thanks to those that suggested in other threads that there may have been a conflict or non acceptable symbol in an appointment that gave me the idea.
Sorry this long winded, but just wanted to make it clear what I had tried.