It may not be that simple, Mr. Vanoord! I'm quite a fan of quality concrete and stainless steel, but it has to be got up there! And it may not be what the client wants! Whatever, we need to consider this one, as indicated by SimplyExploring and his ref. to an on site survey/appraisel. Ad hoc repairs are one thing but serious masonary work is another!
Are you telling me that a Landrover used to use that entrance? Really! No wonder the corner stones are shot! I suspect there was further damage by tip encroachment. I really don't like that big slab, at top right of entrance.
It's in your ball park, SimplyExploring! What would happen if it was in the National Park?
Also, look at Grahami's picture of the adit, in the Cwmorthin Archive Album, image 004. Makes me wonder where all the water came from!