Your joists should be sitting on your dpc and the air bricks should be
between the joists so that air circulates around them.

As for the mush, it may be a reaction from the adhesive in the
chipboard or carpet underlay. Try folding the carpet back, airing the
room and look again in a day or so.

A logical approach to this might help you. Moisture can either rise
from the ground or penetrate via a structure such as a wall which has
moisture on the other side of it. From what you say, your's could
possibly be rising but the chances are it isn't and that the dpc (such
as it is) is working. The only other kind of moisture available other
than plumbing leaks or rainwater is that which soaks into materials
from the atmosphere, - hygroscopic.

Rule them out one by one and you'll be left with your cause. From what
you say it's the latter kind, i.e. condensation. This is evidenced by
the mould in the room itself and the conditions you describe.

What is the sub floor base made out of (beneath the floor), is it just
earth or is there a concrete screed over it? Also, does the
ventilation cover the whole floor or is it windy just in one place?.
Also to check, is it just ventilated when the wind is in a certain
direction or is it constant?

I can't see it being anything other than condensation but that's not to
say you shouldn't be concerned. Condensation is the number one cause
of dampness I see and it can be very destructive.

However, you need to check the moisture content of your joists before I
can advise you further because what might seem wet to you could be dry
as far as fungal activity is concerned. If that's the case, all we
have to cure are the problems in the room itself.

Sorry if this is disjointed, bit late in the day, been up since four
o'clock. If you're still worried tomorrow, give me a ring and I'll go
through it with you on the phone.

xav