I'm not sure whether getting no replies to this (or the earlier) query is something of which to be proud - or terrified!
Am I the only person in the world with this bizarre insoluble problem? if so, why me? Am I a Chosen One or, more likely, someone to be punished for a lifetime of misdeeds!
But there is progress. I have no idea what happened at a technical level or why, but the matter has now resolved itself and things are syncing normally. Until the next time!
Perhaps there is a clue somewhere in the following ...
I run my iMac 24/7 and thus I rarely close iCal on the iMac and when i do it is usually by accident. The same applies to Calendars on the iPad, although I do reboot from time to time.
Anyway, I installed some software on the iMac which required a reboot and, as susual, iCal opened via Preferences > Accounts > Startup items - only this time I was presented with a perfectly blank calendar. A message appeared, from within iCal (oh, how I wish i had noticed what it actually said) saying something along the lines of "importing calendars". This took a little while and it wasn't instant - maybe it does this every time it reatrts but too fast for me to notice.
After this "import", if indeed that's what it was, the syncing was corrected!
Coincidence? Cause and effect? Magic? Too many magic mushrooms for breakfast?
If that is what resolved the issue, then how can I cause an import deliberately for when the syncing goes astray again and just what does an "import" do that a normal launch doesn't and which might have resolved the glitch?
I have had this "import" message before, but very very rarely, so it may well have resolved the syncing issue last time and I just didn't make the possible connection.
Does that help anyone come to my rexcue with an explanation and, possibly, a strategy for preventing or deliberatly correcting syncing issues in the future?