On Sunday, 14 August 2016 21:05:26 UTC+1, Yvonne wrote:
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?

https://www.locksonline.co.uk/Window...h-Windows.html

alternatively a flexible restrictor like

https://www.locksonline.co.uk/Window...ctor-Lock.html

exact suitability will depend on the window frame and mechanism used.

Owain