How to close gap in widows.

…and show the hinge with the window open too.

I have a feeling that there isn't any horizontal adjustment, you might need to add window foam tape to the opposite side of that rubber seal (and tightly close the window).

Agreed.

The Exitex window/door stuff in Woodies is fine, very good adherence as long as the surfaces are cleaned beforehand.

You could cut a length of timber (same as the height of the window) and put it lengthways on the inside of the window that doesn't open so that it overlaps the window that opens, or on the outside of the window that opens so that it overlaps the window that is closed.

The Exitex p-profile on page 47 of the brochure should be fine:

https://www.exitex.com/assets/downloads/New_Weatherbar_Brochure2.1.pdf