The general consensus of opinion is that the hole is an access chamber to a sewer which serves all the houses along the road.
It seems that the steel, or possibly iron, cover was turfed over many years ago and has finally rusted through.
The square indentation in the surface is a bit of a giveaway.
When Sundae brought this to my attention, I thought that my dungeon, nuclear fallout shelter and all purpose man cave had been discovered.
One can rest easy in one's bed tonight.
