OK, an update for anyone else having similar trouble - we aren't there yet, but have narrowed it down and I need to test the theory over the next few days as implementing the solution will be a bit of a pain.
It's taken 3 hours on the phone to Level 2(?) experts at Apple (the ones who step in if the normal advisors can't come up with the solution) - one in Athens and one in Cork (half the time with each) but we do seem to be getting somewhere.
Unless anyone specifically asks, I shan't explain in detail how we got there as it was a complex and lengthy route but we have (we hope) narrowed it down to a problem with corruption with one or more Groups in the iMac's Address Book.
In essence, we exported all contacts into vcards, transferred that file into a new and clean Admin account, imported them into a clean copy of Address Book and the sync'd.
Lo and behold, all contacts (without Groups, of course, as exporting as vcards only copies individual cards, not groups) arrived on the iPad.
The next step is to delete all the Groups on my iMac, but not individual cards, on my "proper" Address Book and then recreate them.
As I have 800 cards in 50+ Groups I need to make sure that I can recreate the Groups that I actually need and, when I've done this over the next few days, I'll test whether the theory works.
I'll report back in due course!