We had a plasterer strip and skim the walls and a decorator paint. The plasterer also put up the curtain rail. When I added the curtains, both mounting brackets came loose and one fell out completely. It is fairly obvious the plasterer had problems because he's used one rawl plug inside another and also filled the space with some kind of white goo with the consistency of chewing gum.
The resulting hole is about 30 mm wide, 80 mm high and 115 mm deep into the wall. Yes, that's right: 3 x 8 x 11.5 cm. It's a red brick wall and that's almost half the total depth.
The interior wall is not plasterboarded.
Inside the hole is quite crumbly. I've removed anything loose with a vacuum cleaner. I'm fairly sure if I were to ferret around with my finger some more would come loose. What I removed is a mixture of old, crumbled plaster and black mortar, plus the goo I mentioned. There is no brick.
I can see the edge of a piece of timber in the hole. It seems it might run the length of the window, about 150 mm above it.
There is a gentle draught blowing from the hole, and the sounds from outside can be heard through it a little.
I have a tube of Polyfilla but instincts are telling me that isn't going to cut the mustard.