The more I think about it, the more I worry that we are being railroaded into installing too much insulation in old lofts.
For the last few weeks, the relative humidity outside has been at or close to 100%. Under these conditions, without heat, anything hygroscopic (like glass loft insulation) ventilated outside will get damp. A loft with 250 mm of insulation basically has to rely on solar heating to raise the air temperature, lower the humidity below 100% and allow damp timbers and matting to dry out.
At night, the situation is reversed. The air outside is cooling down and shedding moisture (dew). On a clear night, the roof faces outer space at about -270°C making it cooler than the air. Water can condense from the air inside the loft.
To prevent condensation, you need heat as well as ventilation. Inside the house, the wall surfaces should be kept 5°C above dew point. If you extended the same logic to lofts, the loft space should be kept 5°C warmer than outside (because dew is already forming outside). I don't think you need to go quite that far as the materials in a loft are different to those in the house.
As mountainwalker points out, most old ceilings do not have a vapour barrier. With a loft warmed by solar heat during the day, the air in the loft can accept extra humidity as it leaks through the ceiling (and evaporates from old cold tanks). On a cold night and 100% RH, this moisture will condense as it leaks.
So, with lots of insulation, a vapour barrier will help. I don't think it is always necessary to install a plastic sheet under the insulation. I'm dubious that putting sheeting over the top of the joists won't cause problems. Various paints on the ceiling can form a vapour barrier. These can be especially useful inside bathrooms and airing cupboards.
