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we have the restrictors plus a lock with small key in the middle horizontal section. Something like the ones in the link but less chunky. Seem to work well so far as a deterrent. Happy to send you the number of our installers if you are SE https://www.locksonline.co.uk/Sash-Window-Locks/Federal-Sash-Window-Security-Lock.html
P.S: apologies should have condensed both replies on a single post
What's the best middle ground? Sash bolts that you unscrew are a fiddle when you want to open the window and you have to remember to put the things back in. Something like https://www.reddiseals.com/product/sash-window-restrictor/ looks reasonably secure although maybe harder to install?