If all that fails, buy some (plumbers mate) which is like a putty, scoop some out of the tub and roll it like a sasuage and apply it to the underside of the plug hole, then tighten it and remove the access. guaranteed to work.
I am going to plaster it myself. Do you suggest i apply 2 thin coats or one thicker coat
Ok....seriously then....it will need a good coat of PVA first......or one thin one followed by a full one if the artex is bare...your plasterer should be able to advise