Free Payment Gateway Button Does Nothing

Hi,

I'm setting up a site and I need to demonstrate the sign up process, so I'm using the Free Gateway however when it comes to the final step, I click 'Subscribe' and the page just refreshes. It doesn't complete the registration or add the subscription.

Any ideas? I can provide a registration URL via email or private message if needed.

PS: I previously posted this question, however it was in a different thread with a related, but not identical query, and have not received a response in a week.