OK, I finally nagged the people at Opera enough for them to take a good look and see what was going wrong. It's actually fairly obvious in hindsight.

Inside the DynarchMenu.addInfo function you set up an event handler for the window.unload event to call DynarchMenu._cleanUp. This is the problem as it seems to remove the info structure needed by the menus. I tried removing the event setup call and I can now use the back button in Opera !! YAY !

The question then is, is it simply just a matter of removing that event or will that cause some other unforseen oddness elsewhere and will the removal cause problems for other browsers.

Cheers,
Michael