Does anyone have any advice on how to repair damaged close studding? - (both the wood and the plaster-on-tile infill) -heavily hacked to take later plaster covering.
Unfortunately once these have been vandalised for later plastering ( once a widespread practice), there is little scope to repair them as you will end up with a lot of work without that patina of age.
Usually here is a straight choise between covering them back up or clean, wax and expose what you have uncovered. This will show history of building.
This will probably look a lot better than new superficial repairs.
As to the infill it sounds as if the original fabric has already been removed. (should be wattle and daub?).
You can repair with lime plaster between beams.
As Middi says, the beams will have been hacked (probably with the edge of an adze) much too deeply to be able to plane out the notches. And if you try to fill them invisibly, you'd better ask your children and grandchildren to carry on the task after you've gone.
Better to treat the adzing as an honest part of the history of the house, clean and wax the beams and learn to love them!