Hi Mark,
A quality outburst of nostalgia, if I may say. ;D
Wouldn't argue hugely, my date guess (again no annotations) was based on Ian, he was about 12/13 when Cyril started to coerced him into potholing with us which would have been mid/late 70's and he looks mid/late teens in the photo, hence my guess. He remembers being at Silverband but I don't think I asked him if he recalled a date, such are my forensic research skills.
By the by on the Smallcleugh archive pics there is one of John McNeill with yet another liberated ICI boilersuit which Roy Fellows dates as 1986 by which time Dad had made off from ICI with a useful retirement package and had taken up full-time mine malingering.
It seems yet another place that has little or no other photographic evidence available, beyond another 10 or so Cyril took, and now smeared away under lots of landfill and drainage lagoons.
Jim