You haven't mentioned heating, I'm guessing there is none. If you're leaving the hatch / access open all the time, the warm moist air goes up there and condenses onto the relatively cold pvc cladding.
I don't know how to remedy your issue, but a quick easy fix would be a dehumidifier, but expensive and a bit of a bodge in the long run, plus it will need emptying a few times a day. You cannot leave it as it is as the condensation might start to affect the rooms below and mould will grow.
In the short term, you need some ventilation, however you accomplish this. Take some cladding off and some insulation to let air in from the rafters, and put the hatch on.
What sort of insulation did you use and how thick? There should be a vapour barrier above the insulation to prevent moisture getting to the cold roof through the insulation. I would also suggest you shouldn't have insulated the floor AND the roof, though I don't know about this.
