Had a slight shock after lifting our carpet - discovered the stairs were riddled with woodworm on top of being completely rotten near the base. Mostly they were being held together with plasterboard.
The stairs were probably the 1812 originals. The base of the old strings was simply resting on a brick tile laid on an open area of dirt. The dirt had been busy letting in moisture to the underside of the stairs (which had been sealed quite recently with some plasterboard).
An emergency carpentry session was called for, along with having to fill in the dirt patch with cement to match the rest of the ground floor (which is about as flat as the austrian alps).