Issues with Pay Per View plugin

Hi,

I'm getting to grips with the Pay Per View plugin, but am having issues when it comes to testing purchases with my PayPal sandbox accounts.

The development site I'm using is http://dev3.digitaldavidson.co.uk/pay-per-view-test-three-excerpts/ returns an error saying 'The link you have used to enter the PayPal system is invalid. Please review the link and try again.' when I try to log in on the PayPal page I'm taken through to.

I get the same error message when I click on the pay with a credit or debit card link.

Does this mean anything to you?

Regards,
Nathan