I would set up the delegate-details.php page to save the form entries as session variables when it is submitted,, then redirect to the checkout page
use Security Assist to set up an access rule to allow if one of the sessions from that page is not blank, then add access restriction to the checkout page that redirects to the delegate-details.php page if not allowed.
that way, when they try to checkout, they will be directed to the delegate-details.php by the access rule until they have filled in the form.