On Sunday, 22 July 2018 18:40:38 UTC+1, wrote:

I'd expect either foam fixing, which I'd expect you to have noticed by now, or frame fixers going through the frame. You'd need to remove the glazed unit to see those.

I am wondering if I might have one of these; a combined cavity closer / former / fixer:

http://archive.profile22.co.uk/wp-co...ser-System.pdf

This particular one uses small camlocks to the keep the window in place:

http://archive.profile22.co.uk/wp-co...013/09/247.jpg

That said, I'm not seeing anything like these cams on mine though!