Sorry, may have confused you! No concrete!
Looking at Graham's pic, I suspect the state of the right hand wall (as you look at it from the outside) is long-standing, ie even if the pile of rubble is removed, it'll still stand up.
However, there are some stones missing and thus the inside face is not vertical - it goes in as it comes down towards the slab it all rests on. If we can clear the rubble (which isn't actually holding anything up), it may be possible to fill in the base of the wall with slabs and a little bit of mortar to bind it all together?
That should cosmetically restore it as well as supporting the base of that wall a bit more?
Incidentally, if you have a look at this picture of Graham's, you will see the very Landie in question, to the left of the adit!
๐Cwmorthin-Slate-Mine-Archive-Album-Image-004[linkphoto]Cwmorthin-Slate-Mine-Archive-Album-Image-004[/linkphoto][/link]
Hello again darkness, my old friend...