I know someone who had a look at it in greater detail.
It corresponds to an adit shaft on the plans, which following inspection, appears to have been sunk on the outcrop of something. As I understand it, site works behind the property involved a water pipe getting severed and filling up trenches which then "self-flushed". This appears to have had something to do with the adit shaft clearing itself. Deep adit is about 120m there, IIRC. She's deep.
The person who had a look told me that the shaft is irregular, and was deep "out of sight". This has self-choked with another fall of earth/fill and is blocked at about 20ft. The walls seem to be largely of fill, although there is a sign of wallrock about 15ft down, however this may not be the case (as it's rather earthy and dirty). There appears to be something on the North side which looks like a working, perhaps a lintel (stone-timbering). It would be suggested that adit access was retained through existing stoping. I gather the adjacent works have hit shallow features, largely collapsed/filled and these probably continue on the strike (associated with the two pits).
I also gather that a neighbour could hear rocks falling a large distance and this sound appeared to be emanating from under her shed (not from where this shaft had opened). Given how high up everything is in the area, this is quite alarming.
There was some quite good drone footage on the interweb.
I'm sure it's all under control, as the people working behind the site are a name you can trust 😃