OK, seems my original post found only ONE possible cause of the problem generating these damned errors, so in a bid to take up the gauntlet, I wrote a small AJAX handler from scratch (again) and discovered one of the main problems causing the problem was using the POST method. Yeah, OK... so that's old news...
On the server, my 'test' page simply displayed all form and query data and this would be sent back to the requestor... Again, nothing fancy there either...
Then I was struck by a number 10 bus... Or at least I should have been... I'd missed something really obvious...
Of the requests that actually made it through and returned the page results... None of them contained any form data...
Checked my code and it was definitely posting form data... So why wasn't the page displaying it?
I had a play around with the request headers and the format of the posted data until eventually, I found a combination that worked...
To save you reading any more and to put you out of suspense... Take a look here...
http://www.perkiset.org/forum/ajax/xmlhttprequest_ie6_ssl_and_12030_error_what_is_the_solution-t442.0.html;msg37