Window frame void space and roller blind installation disaster

[tellme_why]

Hi,
Sixties property, recently installed new window.
Window fitters removed the blind and put it back but screw were loose and roller blind about to fall off.
I drilled a new hole to fit raw plugs but made a mess. I realised the wall above the window got a big void (not an expert).
How to fix this big hole? Should I inject mortar (if yes dunno how to do it) or rather inject expanding foam? How to make it good with lining paper / sanding / painting?

Attached few pictures of the DIY disaster..

Many thanks

 

[blup]

Replacing windows can expose weaknesses/voids in the plaster and around the frame.

Fixing the blind to the top (horizontal) side of the reveal might be an alternative if the plaster there is solid.​

The large hole could then be repaired by screwing, and then plastering over, a wooden batten to the inside of the hole, like on this video (there are many others):

That won't be load bearing so you could try a deep filler if you decide to fix the blind in the same location as before. I have used this (link below) around window frame voids to good effect. It is supposed to be built up in sections but that is difficult when you are applying it vertically. I have managed two/three inches in total depth but it needed time to dry out properly

https://www.screwfix.com/p/ronseal-big-hole-ready-mixed-wall-filler-grey-1-2ltr/95289

Blup