Hello forum, I'm doing some renovations on my flat in London. One of the jobs I want to do is to fix some problems with the old burglar alarm system. I have a sensor on the front door. This seems not to be working because I have constant warning messages on the control panel telling me that there is a fault here. On closer inspection...(see 3 attached photos) it looks like there might be something missing. There is what appears to be a small sensor set into the door frame, covered in a small metal plate. A couple of small wires poke out of the side which used to connect up to the control box (I've temporarily disconnected them because I need access to replace skirting boards). On the door itself, at the exact same height, is a round indentation in the wood (painted blue) with what looks like a couple of old screw holes. This makes me think that maybe there used to be some kind of magnet or something like that fixed to the door...? Whatever it was, it doesn't seem to be there any more, which is presumably why the warning messages are flashing up on the control panel - the alarm system must think that the front door is never closed. Is my reasoning correct? Does anyone know what the missing bit is, and where I can get one? Not sure how best to proceed here and various google searches, looking on eBay for replacement sensors, etc. didn't really help much - I worry that I may be barking up the wrong tree. Grateful for any advice. The local alarm company (which fitted the system in about 2001) would happily come & fix the problem but their call-out charges and hourly rates are eye-watering, so I'm hoping to do it myself if I can. Many thanks, Alan.