Very useful input and thank you.
James, the position of the wood is set by what I am trying to do as the whole wall suffers from the same problem, so moving it is not an option, though I have dome that elsewhere.
I seem to recall using something like Chemfix some years ago but that seems best for an irregular hole caused by "poor" drilling. I ma not sure this will work, though I will try with holes that are now up to 2 inches across and 1 inch deep. Pits, rather then holes, might be a better solution.
Some form of plaster hardening solution might work but I doubt it will soak in far enough, though again I will try.
I have considered constructing a small wooden frame, fixing it to the floor and using that for support but that creates a fairly ugly result.
So far, I think I am stuck with removing a largish area of plaster, back to the stone work, refilling with "colle-ciment" and going from there.
I was hoping there was an alternative but well absence of your ideas working then that is the last resort.
Well the really last resort is to sell the house.