Yvonne wrote:
A member of the family has just moved into a property which has
double glazed sash windows, made from what looks like uPVC.

There appears to be no way to securly lock the lower sash in a
partially raised position so as to provide fresh air.

Is there some mechanism that can be attached that will allow one to
raise the lower sash to different levels- depending on how much fresh
air is required, and then lock it in the selected position?

A stick?