Thanks Martin, I hadn't investigated closely just noticed it yesterday when poking around. At a quick glance it looked like the spare part of the T might have more function.
It's not a dry break coupling. Its a T piece with a blanking plug. Do not undo it when the fuel pumps are running! Its used to drain the tank, you need an aeroquip female fitting with a push fit on the other end that you can attach a piece of hose to