"torge conrad maguar" wrote:
Have just been drilling into a dividing wall in a 1936 terraced house in
London, to hang up a cupboard in the bedroom.

What I'm drilling into is almost like soft sand. I guess it must be some
kind of breeze block mortar that has deteriorated? The only thoughts I
have is to pack some polyfilla around the rawlplug to fill in the hole and
get some grip. Is that a good idea? Novice appreciates any advice on
what best to do. thanks.

Unlikely to be breeze block in a 1936 property, and breeze block doesn't
usually deteriorate with age. You are most likely drilling into the mortar
between the bricks. You can't put filler around a wall plug and expect it to
stick. The easiest and safest solution is to hang the cupboard in a slightly
different lateral or vertical position; wall plugs should be fixed in brick.