It is tragic the open gunnis was used as a tip.
I think the footway "engine shaft" was meant to be fitted with bat access, but was not. When bat castles are built, it's along the same lines as a plug, apart from concrete rings are used through the larger part of it.
If the vent pipe sticks out of earth, the manhole is buried. They specifically left access in the hatch, as I gather the pitwork was in the shaft and was "important remains". There may be a discreet grille somewhere. Probably not.
I have a cage I can rattle about this, but I have a feeling it's "Duchy" and no-one is going near anything for any reason.