I roused myself from my pile of soft things on the floor and rang the landlord. He came round less than an hour later with a friendly workman in tow. They checked everything that could be leaking and found nothing.
It turns out that, because of the lack of ventilation, condensation was forming – not just on the windows but also – on the boxing surrounding the extractor pipe from the bathroom (that had cold air blowing in from outside). It was an excess of this, acting under the influence of gravity, which had woken up Ros the night before.
Further to this, the only way to counter all of the above seems to be to open the windows and turn up the heating. The friendly workman is going to fit (or already has) a hinged flap to the exterior of the vent to prevent cold air blowing in. We’ll be making an effort to not use the room for drying laundry (it has the most accessible radiator for standing a clothes horse in front of). We’ve also rearranged the bedroom so that the bed is no longer directly in the line of drip – this involved taking everything apart, emptying the wardrobes and moving a giant shelving unit full of comic books into the spare room, and then putting it all back in a different order. It looks very strange and different and we’re going to have to keep an eye on the boxing, but if we don’t get woken up by sudden, unexpected facial wetness in the future then it’ll be a day well spent.