Looking again at the first two images I would guess the portal was extended to bring it to the level of the general area, sort of 'heapstead' thing.
Could be my dating was out as there are some similarities with Manxman 2012 pic of the "New Decline"
🔗78239[linkphoto]78239[/linkphoto][/link]
Perhaps this development was done by Weardale Mining and Processing, they seemed to have run the site from 1982 to 1991 but given chriscambo's date of the late 80's for Mr Wilcock's photo and the site being cleared by 1997 (Manxman's pic was prior to that - edit -maybe 1992-6 from one of his other pics?) things still don't quite gel, but getting closer.
Jim
PS re. above pic of HMP, North Pennines AONB have a picture of him and at least on other PDMHS type at the horse level intimating that they are fluorspar miners:o