get 22mm and plane it down.
Get 18mm and pack it up off the joists with some 2mm strips of wood.
I've gone with the latter Got the 18 and will wither get wood, glazing packers or something to make up the space.
So last night I took the radiator off the wall for the plasterer who has come today and cut a board out of the plywood sheet. Then I ran out of time