Bongo Fury

So firstly the factory plastic is still on the bit you lie on in the roof section which I've never taken it off as haven't felt the need too. When I sleep up there I have a velvet type material covered foam mattress and the Bongo sheep skin type thing on top off that, then in a sleeping bag. I also have the Bongo inner tent bit which brilliantly keeps condensation off the top of the bag.

The weird thing is if I lift the bag/sheepskin/mattress up it is wet underneath the mattress and on the plastic. There is no dampness between any of the other layers whatsoever.
I always thought condensation was when hot "air" touches a cold surface and condenses but there can't be any hot air reaching this point as it's totally covered.
How on earth is it getting damp there? It not sweat and as far as I'm aware I haven't wet the bed

Difficult to work out quite where the moisture is without a picture.
Maybe the moisture is coming in from underneath the lower tent frame ? The lower tent frame is held to the roof by scrivets and a simple strip of window sealing type foam forms the moisture barrier between the frame and the roof, on mine the seal had failed over time and was letting the water run thru under the frame and into the inside of the AFT. I just removed all the scrivets, removed the old foam strip from the lower edge of the frame and then replaced the foam seal - however rather than sticking it to the frame I stuck it to the roof itself and then lowered the frame onto the foam and replaced the scrivets.

Condensation is a complex phenomenon. One thing for sure is that polythene is a perfect moisture barrier: it doesn't let moisture in or out. I suspect your breathing is the moisture generator. As the moist air cools and falls, the moisture will tend to condense and may wick via the velvet / foam etc and end up trapped against the polythene and builds up. Hard to be sure without being there.

As Mike says
That will be condensation
Is it there when it's not raining

Sounds very similar to our experience with a trailer tent where the beds hung out of the sides on wooden "wings". We were surprised to find moisture between the mattress and wooden board though we slept on a similar set of layers to yourself. We could not find any leaks in the canvas so we concluded it was condensation. As an ex-mountain bloke I put closed cell foam insulating mats (the proper expensive version and predecessor of the self-inflating thermarest) under the mattress which significantly improved it.

Your sheet of polythene will show up the slightest collection of condensation.
So if there is any reason for the base under the polythene to be cool then it would not surprise me if it is condensation.

Almost certainly there will be some body warmth traveling through the bag then sheepskin thing then mattress but I'm surprised as the plastic is actually wet to the touch and I have to dry the mattress out. It's definitely not coming from the outside as last week it didn't rain when I was away. It's a strang thing though because it's not like there's loads of air under 3 layers of textiles to condense.

I think the source is most likely exhaled breath and loss through skin.

https://www.quora.com/How-much-water-do ... -breathing

(note 28% is through insensible loss - hopefully the others dont apply!). As the exhaled air cools, it falls to ground and the moisture condenses out of it. Similarly, skin moisture loss will migrate through permeable layers until it meets a cool / impermeable layer where the (already happening) condensation will change from diffused dampness to clearly visible droplets.

That's my theory anyway