Insurance will pay Dave. I would expect it might be pitch fibre pipe which often collapses. That was used in the 1950s and 60s.
That can be a problem with random companies building conservatories over drains.
One was built in my road and the manhole is inside the new building and covered over with a wooden floor. That drain cannot even be inspected or rodded now. The so called builder sealed the lid down with silicone bath seal so no smell came out.