very large gap between window frame and wall

[mishmash85]

Recently moved in the house.
There's always lots of noise coming through, so I decided to tidy up all the sealant etc as it was a very poor job.
I removed the top trim piece (it was cut way too short) and came across this large gap.

I understand there might be some small gaps, but this is 40mm which I find excessive.
Or am I wrong?

What can I do to make it more air/water tight as I suspect heat will be escaping.

I know it's possible the frame is too small (the window is 1900mm long)
But until I can have new windows installed I'd like a temporary fix.

 

[crank39]

Expanding foam, trim it flat and reapply new trim

 

[mishmash85]

Thank you, I did consider expanding foam, but wasn't sure if it would be suitable, or maybe other options.

I have inspected it more closely and found the reason for it.

The lintel above the window hangs lower than the gap inside, so the frame could only go as high as the lintel meaning the unsightly gap.

So perhaps a new frame wouldn't make a difference.
I'm guessing I would need to reduce the window opening approximately 35-40mm on the inside.

What is a good fix for this?

 

[foxhole]

Trim often used to hide poor fitting or when undersized replacement windows are fitted. You can foam and fill top surface back to match surrounding, less important over top window.