We all love sash windows but they are not the securest things!
Sash windows are a puzzle. First of all it's a daft name; literally it pretty much means a window window. What this usually means is a double hung sliding sash window. It's also puzzling as to why they are so popular. They have almost no redeeming features. They are always draughty, horrible to maintain, and they are difficult to secure.
A simple catch that most sash windows come with is completely inadequate; you definitely need something more. The most secure is a bolt that goes right though both sashes, however, the problem is that as time goes by, the sashes and frames move and change size and within a year or two the bolt holes will no longer line up. This is especially true in a place like London where subsidence is commonplace.
Sash stops are pretty good; they prevent the window from opening more than a few centimetres unless they are removed using a key.
get in touch with Locksmith Southampton if you would like more advice!