Several types of sash locks are available. The one I like best are actually called window bolts.
Sash bolts attach to the top sash and the protruding bolt stops either window opening. They are very reliable, quick and easy to fit but are also very flexible solution as you can position the bolts on the sash so that either the window won’t open at all, or open to say 4 -5 inches. The latter allows you to have a window open partially to air the room which is especially desirable in summer months.
To fit the lock, mark a suitable position on the window sash. If you need to cut around a bead on the window frame, mark the top and bottom of the lock, open the window, then cut with a saw. Remove the section of bead.
Close the window again and hold the lock against the sash. Predrill for 1 screw hole and screw the lock into place. Then check the alignment, predrill for the other screw and secure to the sash.
With the window closed, push the pin through the hole to mark the position of the hole in the window frame. Then drill a 9mm hole to just take the length of the pin. You should not drill in too far or youâ€™ll come through to the other side. Remove the sawdust, test the pin and snap on the window bolt cover.