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.
7 Make sure you make the gap too small for an intruder to get in and also if you have small children too small for them to fall out from. (approx. 100mm.) They are removable with a special key only.To get the depth right when drilling wrap a bit of sticky tape on the drill at the depth you want.You should find when screwing the bolt housing into the window frame it's a tight fit. If you find it very difficult use a little dab of washing up liquid to make it easier.
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.