Our existing DG window handles have a halfway point so we can put it on the latch and still lock the window. You'll need to speak to your DG company, doing it yourself might invalidate your warranty.
the trouble is, if you leave it open enough for someone to get a lever in, even if you lock it then people could put an arm through and wrench off the lock.
The little vents in the DG frame give me enough ventilation, have you got one of those?
I am so pleased I found this question as I live in a bungalow and have the same trouble. At night could do with them open a few inches but need something that is lockable. I will get some as well.
mallyh yes that is great........or will be when it is warm enough to open the windows at night.! lol. can you let me know how you get on with them please.? Many thanks Brenda x