The leak is now sealed (I did it just before the 5 weeks of no rain :) ).
Where the water dripped onto the plasterboard, a section about 12" square has been slightly pushed down and broken through. The ceiling was papered over, and I must have done a good job, as the paper is intact apart from a slight hole I dug to drain the water.
Is there scope for somehow cutting out the damaged section, and replacing it with something like plywood ? The cosmetic finish doesn't have to be 100%, so I am thinking of slicing the stretch of paper involved out to undertake the work, and then restick it.