You cannot rely on existing central sash fasteners. Sash stops are strong and convenient to use and do not need to be removed from window when opening fully. They can be set into the top frame allowing the window to be left open about 5″ for ventilation but still secure. For optimum security fit in pairs.
An alternative form of lock is the dual screw. These in effect bolt the two sashes together. An added bonus is that, in so doing, they also reduce draughts.
The beading which holds the glass in place is frequently only pinned. The window can be made more secure, either by gluing in addition to pinning or screwing the beading, if wide enough, in place.
A modern style is the ’tilt to clean’ sliding sash, not only sliding up and down but also tilting inwards for cleaning by undoing two clips on top of each moving sash. These should be secured by fitting both sash stops and dual screws.