Ok I've narrowed down the problem using developer tools in both chrome and IE and looking at each request step in detail. The problem is being caused by a new PHP session ID is being generated in the checkout/cart page - but only in internet explorer. So session cookies are being ignored or lost for whatever reason on the redirect.
The same problem does not occur when I use the demo.abantecart.com site, the same PHP session ID is retained through all stages of the product add process and the cart is correctly populated.
Would it be possible to get a sanitised copy of the httpd.conf being used on demo.abantecart.com perhaps there is some setting being used by default in apache (and IIS) that is causing the problem?
I can post logs of my network inspection to show the issue if that will help diagnose the cause?
Looking at the code I can't see where session_write_close() is being caused before the redirect, could that be the cause as per
http://stackoverflow.com/questions/8071006/ie-not-passing-php-session-variablesPlease help me here developers, I've spent weeks here trying to rule out everything I can think of and I'm truly stuck. I'm a system admin by trade I'm not a novice and I've isolated every component I can think of. You've been very helpful with any other issue I've had but this one is so major that I cannot use your cart solution if I cannot get a resolution.
I'm sure other people would come across this given I've got a very standard out of the box setup.
Please help!!