-
If that's an outward opening window, with friction hinges and the gap is in one of the hinged corners, then definitely new hinge time. If the gap is on the side or the two corners with the locking mechanism, then the mechanism isn't pulling the sash tight against the frame... or the sash has been pushed across by the the action of the locking mechanism movement.
If it's a tilt and turn, it probably needing some adjusting.
Any recommendations for uPVC windows/double glazing repair in South London (SW2/SW9)? Got a gap between window and frame which I think means they might need new hinges