My thanks to everbody for their suggestions but might I clarify a few details?
The PIR is on the front of the house and windows are not involved.
The CH flu is at the rear of the house and emerges through the roof, so is several feet above the PIR which is below gutter level. Also, during recent problem periods, it has been on the leeward end of the house.
The PIR (and lamp) are angled downwards so as to prevent nuisance to neighbours and false triggering from passing pedestrians (I can walk along the footway close to the low front wall without triggering it).
Heating of the wall is not possible - when the sun comes round to the front of the house it only shines on the outside of the wall.
There are no trees or bushes within range of the detector, only some small shrubs which are reasonably wind resistant.
Spiders? Wouldn't they cause problems irrespective of whether it is windy or not? The PIR is fairly new and, as far as I know, is well sealed (but I will inspect it closely in daylight.)