I have managed to get 2checkout working to a point where user signups are now processed, and membership levels set correctly.
My remaining issue (for now) is that the user is not redirected to the correct URL on completion.
Within 'Membership' option you specify the redirection URL on successful payment:
When a user has signed up for membership and completed any payments required, they will be redirected to this page.
However the 2CO gateway requires you enter http://siteurl/paymentreturn/twocheckout to be entered in the 2Checkout site admin for membership to work.
How can I get the user redirected to the correct page on successful completion?