RPCodie

Last October (2017) I replaced 3 oldish fire/smoke-alarms with 3 Kidde Firex KF10. All have functioned correctly until one day ago, when the alarm at the top of the stairs within the loft extension failed to show the green LED. I checked the local fuse box & no evidence of any tripping, and the lights (on which I assume this to be connected) were working correctly. I have removed the alarm from the ceiling, but cannot see any obvious fault, & have checked the wiring back into the ceiling for loose connections. Depressing the "hush button" gives the necessary short alarm note. the other 2 alarms have the green LED on. As a red herring, my wife heard a distinct "clunk" around the time the alarm failed in the area where the alarm is situated, and temperatures have been pretty high in the roof space. Any thoughts/ideas/suggestions? Many thanks