I have installed a motion sensor light in our dowstairs toilet (thanks,
but I've already made all the jokes!).

It is apparently a good quality one - a Steinel from CPC
(https://secure3.farnell.com/jsp/ende...sp?sku=LA02529)
It is in effect a straightforward ceiling light with the PIR neatly
built into it.

I've been battling with the settings for a couple of months now. What I
don't understand is how, when the room is effectively a sealed chamber
no more than 3 foot by 6 foot, with no windows and the doors shut, it
should come on by itself. Our bedroom is directly above and I can hear
it clicking on in the middle of the night. I can understand it may be
sensitive to sharp blasts of warm wind for example (haha, again), but
in the still of the night? Whatsmore, the sensitivity is turned down to
such an extent that my 6 year old has to jump up and down like a
lunatic to activate it!

Any ideas what might be causing this? I could of course just replace it
all with a regular lightswitch, but where's the fun in that?!