I concur with this … it's the insured's responsibility to ensure that windows are locked and not left open. Open windows do not enable a 'forced entry' … they're an invitation for an 'unforced entry' which could be an excluded risk in the policy wording. And all windows should be fitted with insurance company approved locks. This should all be documented in the small print of the insurance policy. Insurance companies will only pay claims in accordance with the policy … and will exclude any claims where the policy terms and conditions are contravened. Regarding the expression 'get-out' … insurance companies will usually help their policy holders and try and ensure that they are fully covered by drawing the insured's attention to the policy's terms and conditions. If an insured does not comply with the policy's conditions the the claim is invalid. I'm a former insurance professional.
Your first port of call should be the police - especially as another dwelling has also been burgled. Forensics science and technology might be able to help identify the intruder. From your description sounds as if you might have some video footage which could assist a police investigation?
And it's sensible to have a secure safe installed. One of our forum members has a home security business … used to be based in High Barnet.
dunk