Sorry the key thing I should have added that this pir is wireless. There are a few more like this in house that don't false alarm though.
As I said many years ago when I put in a system in my previous house, if you took all the guidance from texecom you wouldn't be able to put these pirs anywhere because they have to be a certain distance from steels, radiators, windows, air, and walls! The biggest problem for me in placement are steels as both ends of my living room have steel lintels. Also both ends have floor to ceiling glass windows / patio doors although I can't remember if they are not meant to be facing through a window or not.
Could a placement issue be so intermittent?
Many thanks for the reply.