Hi Debs,
We have a 2 bed in Jardin 9. When we snagged our apartment I also noticed that the window in one of the bedroom was draughty.
PW came out and fixed it. All it was was the draught proof strip was missing.
check to see if this in place on the frame where the windows overlap. Its a fur type strip.
If its missing you can get some from ebay.
Phil