It sounds like you need to replace the "Espagnolette Window Lock". Check out this link:
https://www.upvc-hardware.co.uk/locks-gearing-163/espagnolette-window-locks-186/
They seem to have a good selection of upvc window locks to choose from.
If the link doesn't work then go to the homepage www.upvc-hardware.co.uk
Chris White
March 2008