With the handle removed, use a small metal hook to go into the square cracked hole and turn the hook in the same direction the handle used to move to open the window.
The lock inside the window works on a rack and pinion arrangement. The pinion is the square hole where the square bar of the handle fits into. The pinion has teeth which engage into the rack of the lock to move and lock the window. Once these teeth are broken the rack will not be engaged so will not unlock. The metal hook acts as a replacement tooth to pull the rack to the side and unlock the window. Hope this makes sense.
If this doesn't work then try gently to gemy the frame open from the inside with a wide, strong but thin blade (a strong paint scraper maybe)
Once the window is open you could even buy a replacement online, this company seems to have the best selection on the net:
www.upvc-hardware.co.uk
Direct link to window locks:
http://www.upvc-hardware.co.uk/locks-gearing-163/espagnolette-window-locks-186/
CJ
April 2008