The fixed leaves of the hinges are recessed into the outboard edges of the reveals which are of softish (Chilmark) stone. Individual, roughly square, holes for each fixing screw have been chiselled out about 3/8 to 1/2" across flats and up to 2" deep (somebody must have had a lot of patience!): these have been plugged with wood into which #10 or #12 woodscrews up to 1 1/2" long have been screwed for fixing the hinges.
A lot of the plugs no longer hold screws and there are some cracks in the stone between holes so I don't want to use anything that expands and is likely to brake the face stone away - it's happened on one already and somebody has replaced it with an ugly backflap fixed to the face of the wall, further up than the original hinge.
One thought is to plug the holes with Plastic Padding Liquid Chemical, spot through the hinges for the hole centres and then drill and tap M6 and use countersunk M6 screws instead of woodscrews.
I think that that should work but there must be other and possibly better or easier ways of doing it.