On Monday, November 27, 2017 at 3:40:27 PM UTC, newshound wrote:
On 27/11/2017 09:36, DerbyBorn wrote:
I would want to stick some plasterboard in first.


As I suggested. One option, if the OP has some stale finish plaster, is
to glue the plasterboard in place with that, and in half an hour it will
be set solid and can be finished with a couple of millimetres of
one-coat or lightweight filler. Plaster of Paris also sets pretty
quickly. I guess both stale finish and PoP may still move a bit after
they set, because of drying out.

Whatever you do, it does need to be fixed securely, and preferably with
some hidden mechanical keying, to discourage the weighty lump from
falling out eventually, as has already happened once. Lots of potential
ways to do this.

2 part Profil from Toolstation goes rock solid almost instantly. Very handy for odds and sods like this