David Johnson has kindly been up there for a look and he says the following (reproduced with his permission):
"This morning I finally trotted up to the mysterious stone feature that has been exercising folks' minds. It's precise location is SD73531 80174, by the way. It has definitely been narrowed, at least on the left side entrance, and looking at the ground above it, behind the entrance, it was clear to me that it had been dug backwards, from the surface downwards, rather than having been tunnelled in. I also noticed just to its left a pile of flagstones that were stacked there rather than having naturally landed there. There also seems to be a ramp leading up to the top of the passage from the little path a couple of metres to the right of the entrance. I would say the ground was taken up for at least 6m from the entrance.
I stand by all the possibilities that I discounted in my previous email and I now also discount my hypothesis that it might have been the offtake for a leat serving Top Farm. The steepness of the slope and the mass of scree below rule that out. What I can say with total conviction, using archaeology-speak, is that it's a truly excellent example of its kind. You'll be pleased to hear that.
While I was up there I looked (again) for the source of the Broadrake leat, and I found it and was able to trace it all the way down to Broadrake. If either of you would like precise details, let me know.
Oh, I forgot to say (!) what I think the feature is. I never make something up or try to fob people off with spin. If I don't know, I say I don't know. For this thing, I haven't a bloody clue. What I will say is that on my way down I wandered left and right looking for any trace of a leat going downhill to the south-west that might have originated at the 'thing'. Initially nothing, but over the centuries especially up there features can be dramatically changed as we all know and it is just possible (no more than that) that there may have been a leat, following what is now that wee path. It's odd that at the first wall you meet going downhill, next to the gate, there is a water smoot. Yes, the wall has been rebuilt to insert the gate but the massive lintel on the downhill side of the wall looks original to me - why would they have inserted a smoot when that wall was first built unless it was to allow a cut channel to flow through? (And I don't think it relates to the modern drainage ditch on the west side of the path). Further down the grockle path, maybe half way or so between the two gates, there is a straight channel that has clearly been dug contouring along and again not linked in any way to the modern path. It has a very pronounced cast bank alongside it and is fed by a number of short rills that all enter it at right angles. No way natural. Below there I lost it - too many little channels all over ... until I picked up the definite Broadrake leat."
He also suggested that more could be learned if a camera could be got in there, to give a better view of what lies out of sight. Does anyone fancy a go at this? Maybe a GoPro & video light on some sort of wheeled platform, on the end of some drain rods? Perhaps even a tiny drone with anti crash capability?