How do I fix this hole next to the window ?

[ihatealldiy]

Hi,
I've got a hole where the Venetian blinds were fixed by the previous owner and find that the edges near the window are quite hollow and a regular fixing wasn't used to hold one end.
Now, on clearing the small debris, there's a hole at least 4 cms deep and 4 wide. Don't want to explore this further if I can help it as I found similar problem in another window.
Can I use Polyfilla advanced to fill it up and then hope to use plaster fixings for the blinds? Polyfilla says holes up to 5 cms deep can be sorted out. Should I use anything else before the filler to hold it in place ? Any steps for a newbie ? Thanks

 

[Wayners]

Options.
Make bigger to see what's there and fill after having a thick about it.

Looks like plasterboard? Rip off and replace with new.

Other option is cover area with new plasterboard.

Or PVA. Use wood glue if you have some and fill with poly filler.
When first fill is setting fill again to build out if too big to do in one hit