The next morning we gingerly lifted the slab again: we did it gingerly because the concrete in its cradle was cracked badly and looked about ready to give up the ghost. The cradle didn't look any too safe, either, being almost rusted through in places. Setting it aside, we looked into the cavity. There, about a foot below ground level, was another concrete slab. This one was round, and set on top of its hole so there was something for us to grab hold of. We were nearly overpowered by the smell as we lifted it, and we very quickly put it back in position, but not before we had seen what was underneath.
The dark, swirling mass was the content of a septic tank, not withstanding the fact that the estate agents' particulars had stated that the house was on mains drainage.
On the premise that discretion is the better part of valour, we replaced the top slab and went back to scraping the hall floor.