endymion explained :
I need some help with this because my husband is busy and hasn't got round to
it. he moved the storage radiator ( on my request) and behind it he has left
two holes which were cut into the plasterboard walling and hidden until the
heater came off the wall. I think they were from some previous wiring attempt
there. I cant say for sure. . One is four inches square and the other about
six inches. How can I fill them up? This needs to be an easy botch because
I am no good at DIY I am fed up of the gaping holes. My husband "filled"
them temporarily with kitchen towel to stop draughts. behind them is the
cavity wall ( I think) - well its a hole there anyway.

Get hold of two pieces of plasterboard which are big enough to overlap
the holes by 2" on all sides (not critical), then thread it through the
hole so it covers the back with a good overlap -remove any loose bits
from the edges of the original board first, so it can fit tight behind.
A loop of wire through each piece will make it easier to hold them in
place. Screw some suitable screws around the edges of the original
board, so they can pull the new board tight up behind it, but do not
tighten them up fully - so there is a gap.

Mix some very thin plaster, spread it in the gap between the two layers
of board then quickly tighten the screws to squeeze the plaster out.

Now remove the wire, mix a normal plaster mix and fill the hole up to
almost level with its surrounding wall (1/8" below) leaving the finish
rough. Allow that to set, then put a skim coat on top carefully making
it smooth and level.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk