Void between concrete lintels

[Rickrick]

I had some builders in who cut a window out for me, they have used a Concrete Lintel and put plasterboard and skimmed it over. I am now putting up a blind and looked at photos before the plasterboard went on that there is a huge void between the two lintels. Ideally, I don't want to take it down and start again so I am just wondering if there is something that I can squirt in (like they do when you have cavity wall insulation) that will fill this void and also take the strength to fix a blind upon it? Or any other suggestions would be greatly appreciated. Thank you in advance

[OchAye]

there is a huge void between the two lintels

What two lintels? You said a new window opening was made and a (one) lintel was fitted. Where is the void? If you have photos post them please.

[Rickrick]

This is the photo before the plasterboard and the skim having installed the window

[darrenba]

What is that white stuff above the lintel? I hope it's not expanding foam. Have they installed a cavity tray?

[cotswold builders]

Should be a catnic type lintel with cavity tray, packed tight with sand/cement/packers and not foam.
It needs coming out and doing properly