I can't test purchasing from your site because the action of the checkout form on the checkout page has been changed to point to paypal instead of pointing to the confirm page.
On the checkout page, change the form tag:
<form name="checkout_PP_LI" method="POST" action="https://www.PayPal.co.uk/cgi-bin/webscr">
<form name="checkout_PP_LI" method="POST" action="confirm.asp">
the checkout page should not be posting to paypal, the post to paypal should occur on the confirm page.