Ended up following that code as per your recommendation and the checkout process ended up back at the success page, with ...
Notice: Undefined index: OrderID in /clientdata/n5200-2-dynamic/b/u/buywineonline.com.au/www/checkout_success.php on line 160
... at the top. However it was followed by the contents of the shopping cart, so I'm a bit confused as to why it remembers the contents of the shopping cart but not the OrderID.
Any thoughts there?