Thanks for the feedback, The dining room where the PIR was triggered is about 17' by 13' with high ceilings. As you can just see by the photo, I placed it in the corner of the wall the window was on looking into the centre of the room. The only reason I fitted 2 quads was that Texecom did a package that was considerably cheaper than buying individual components and came with 2 Quads and 2 Dual's. I had to buy 2 extra PIR's and chose 2 more Dual's as price difference wasn't great. I deliberately picked what I considered to be the 2 areas least likely to be triggered by 'hostile' conditions: landing and dining room. The other PIR's are in Kitchen, living room, hall and garage.
The alarm went off last night about 5:30 am, just as it started to get light.