I don't believe this is a significant problem unless you have
lots of cracks or holes through your plasterboard and an
unventialated loft.

The problem is that at 21C, the air can hold a lot of water. With a well
ventilated loft, at, say, 5C, it can hold considerably less. Plasterboard is
not gas proof, so the air rises up through the board and into the loft. As
this air cools, the humidity rises. This water has to go somewhere. Without
a vapour barrier, it will dump itself out as soon as relative humidity
reaches 100%, which will be inside the insulation. The solution is to make
the system completely gas tight on the warm side. This is the vapour
barrier.

I think this is well OTT. Also, I would not put a vapour barrier _over_
the loft timbers. Seems like a recipe for them getting wet and being
unable to dry out.

To keep the wood good, it needs to be either warm (i.e. under the insulation
and thus part of the house's heating envelope) or dry (i.e. above the vapour
barrier). This condition is satisfied when sheeting over the joists. If you
use foil backed plasterboard, then it is both warm and dry, which is even
better.

Obviously, the vapour barrier prevents water escaping from the house,
raising humditiy there, instead of within the insulation. The solution to
this is ventilation. In a well sealed house, heat recovery ventilation is a
good idea, as it saves pumping expensively warm (but too wet) air out.

Christian.