That is shocking...but sadly typical.
The MultiMap aerial image ‘seems’ to show a semi-circular pale feature in the right place. See:
G
Grendel
There may be a fourth chamber.
No muddy digging needed – it is there.
The site you talk of exists and is the subject of an article in the current (issue 90) of Northern Earth. It is called The Bullstones.
Me thinks Burl did not actually visit this site.
It was there and complete less than 50 years ago but...