Error code on final stage of payment.

Hi folks, Im trying to get my payment working but I get an error on the final stage of checkout.

Any ideas how to fix it? Ive contacted the developer of the extension im using and he hasnt got a clue why its happening. I use Sage Pay for payments.

Thanks.

The checkout only has Billing info, how do I make it so it shows billing and delivery? or at least the check box to say billing and delivary are the same?


http://www.killercomics.co.uk is my site if anyone wants to have a look?