Thanks for the email Scott. I'll try it with a live site when my DNS allows me access.
Due to having multiple sites all my development work is done in the IDE and then uploaded using a separate ftp program, so I'm hoping it's not a conflict issue, otherwise I'll be back to version 3.5.
Thank you for your assistance today.
Just an FYI. The PayPal, Google Checkout and Authorize.net modules have not changed at all for the past 3 versions. The code that you use in 3.5 is 100% the same code that is in 3.6. With that, I think the issue is probably related to something else.