Thanks Spr0cket, sound advice which
tallies with the research I have done so far. Don't really want to hack off up to a metre and start again, but I know it's time and money down the pan if I take a chance, do a localised repair and it leaks further up the wall. I'm certain that the problem has arisen due a faulty seal around the trim as the plasterboard was only really wet in the corners and ...