Hi Grif,
I am not sure I fully understand what you are trying to do, but I will attempt to answer your question. There isn't really any customization you can do from within Method to have the customer go back to your site after they logout of your Customer Portal. However, I did go to your website, and noticed that you when your customers click on the "Customers Portal" button (from your sites Travel Plans tab), you are opening up the portal in the same Browser Window. If this is what you are referring to, I would suggest updating your website code so that when the Customer Portal button is clicked it will open up the Portal in a new tab. This way, the customer will still have your website tab open, so when they are finished with the Portal and close the tab, they will still have your website open.
- Ben