You need to clout nail some metal lath over the wood in the walls and get lots of pva into the surrounding plaster. Then mix up a thickish mix of bonding and get it well into the cavity. Apply it bulbous and then take a straight piece of wood and rule across/up the area using the walls each side as a sort of guide rail. It doesn't have to look pretty but it will be true with the rest of the wall. Come back the next day and knock off any high bonding blobs etc and lightly score. Give the whole wall plenty of sticky pva not too watery. Come back in a short while and pva just the bonded areas to help with the whole wall going off at the same time and replaster as normal.