Unfortunately, leaving the hatch is only half a solution. It will help with the condensation problem, but also increase heating bills.
Because the roof was laid with older type non-breathable felt, the vapour has nowhere to go. Your problem would be solved instantly with good roof ventilation. No heating required.
Two ways of doing this ... crossflow ventilation with vents fitted on both sides of the house in the soffits (flat bit) just below the guttering is one way.
Probably much easier would be to fit several slate vents to both roof slopes. They are simply slate-sized vents replacing some of the ordinary vents.
Your landlord should see the sense in this. If your things are being affected, then think what it's doing to the roof timbers.
Good luck with convincing him :o)