Well, I was back there again yesterday & today. The shaft has now been opened up completely. They wouldn't let me down because their Health & Safety guy was on site, but I managed to get a pic. This shaft only goes down 21 Ft. & the base is solid sandstone. At the bottom, from the corner nearest the alley & on the Holborn Hill side, a passage goes towards the different concrete in the alley (shown on page 3 in a post by Waddi, where he pinched my pictures)
This is the bottom right corner in the pic. From there, the shaft goes down to the bottom, via 3 or 4 landings. The theory is that the shaft was sunk from the alley (which is where I remember a building) & then the lateral tunnel & the shaft we can now see were built. This is where Pinzgauer remembers it. When I remember it, they were still constructing, because I remember hearing explosions. When it was completed, they capped the shaft in the alley, to restore the full width.
My contact wasn't there today, but he's promised to get some pics for me. One of his mates said that after I'd left yesterday, he went down all the way to the bottom. This was perhaps not as risky as it seems, since from the first breakthrough on Friday, we could detect a draught, so I guess there must be air bricks at Holborn Square. Rather him than me though.
I'm attaching some plans; the one I've written on shows the arrangement. The others are some old plans which I couldn't make sense of before, but now I think it shows the passage dotted.
If my contact sends me any pics, I'll post them here.