Fill the gap with bent chicken wire with newspaper screwed up as a core inside it. Push it into the gap in the coving so it's held relatively snugly and does not protrude below the level of the coving.
Difficult bit - make a moulding template as close a match to the existing profile as you can - thin moddlers plywood can be cut with scissors.
Slap on a base coat of plaster of paris as rough as you like but again not protruding. Build up another layer and use the profile gauge template to make the new matching profile. Don't try to do it in one or two goes - take your time.
Smooth off with fine sandpaper and decorate.
Much more difficult with decorated coving...
Good luck...
Peccavi
October 2009
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