pro sites is set in an upgrade loop and wont check out

I have a site:
http://andyfielder.co.uk/pro-site/?bid=57

Where the user is unable to upgrade and is stuck in a loop where they keep coming back and are unable to upgrade. I have now turned off the auto redirect and cancelled the trial period, but they still cannot upgrade.

Is someone else able to assist, its not the first issue I have had with pro sites recently.

thanks,