I've ordered a couple of Continental Four Season 28s for the Longstaff. We've been running Marathon Plus all winter (25 front, 28 back); they're slow and heavy, but we've not had any problems with them - the only deflation we've had was a valve seat failure on the Upper Thames last November, so they've served us well for over 3500km of wet, gritty, winter rides.
As a tandem team, we've shed getting on for 20Kg since Christmas; our combined weight is now within what one might consider reasonable limits for a lighter weight tyre, so I think the time is right to switch.
If you see us fixing punctures in the next few weeks, you'll know it all went horribly wrong!