When you get a gap down the hinge side of a window, it is usually because one of the window hinges has broken. You will need to get hold of a new pair of window hinges (proper name: friction stays) and replace the old pair.
I work for "safe n sound ltd" and we supply hardware for the repair of PVCu windows and doors. We should be able to supply you with whatever you need in order to repair you window. You can identify your product and buy it using our website "www.safensoundltd.com". You can also contact us on 01422 378558 9-5:30 mon-fri for further assistance.
Chris White, April 2005