Okay, I've read the replies so far, and followed up on replacement
parts. I appear to have non-standard windows. Mine have espagnolette
mechanisms, the handle has no tab to rest on the frame and hold it
closed, it has a moving plate on the casement (?) with pins that engage
in the frame when the handle is turned.
I've found something, at what I consider a reasonable price, here:
<URL:
http://www.double-glazing-parts-repairs.co.uk/window-lock-genesis>.
The shortest length on this site is 400mm, that seems a bit long for
mine - will have to wait for daylight so I can see to remove mine and
measure it.
If anyone has any recommendations on sources for parts like this I'd be
grateful to hear from you.
Justin.