Seems to be almost there and I see what we're doing.
A valid Captcha enquiry still works OK but an invalid one is going a page that doesn't exist:
It's going to EG domain.com/123 (IE it's missing ItemID=)
Just top confirm, the redirect code near the top of the page I'm using is:
$WAFV_Redirect = "Items_Detail.php?invalid=true&ItemID=".isset($_POST['ItemID'])?$_POST['ItemID']:"";