Old Nick wrote:
(1) I have just purchased a wireless SecurePro burglar alarm. In an
unusually rash moment (we had been broken into for the 3rd time) I
bought without my usual obsessive research. Needless to say I now find
I am not happy. Reponse from the vendor has been umm...

(...)

These guys? http://www.securepro.com.au/

Short suggestion: Place the PIR unit a little below eye level.

Long suggestion:
Have a look at a thermogram of a person wearing a jacket in a cool
environment. You will see that we present our sharpest, most consistent
heat contrast from our face temperature because we normally do not cover
our faces with insulation.

Background:
The lens in front of the actual sensor breaks the IR
into vertical stripes. As one walks in front of the PIR, several pulses
are sent, not just one. The PIR is a sort of pulsed DC device.
It presents a decoupled, differentiated output.

Any infrared sensor relies on a sharp contrast in heat level between
'person there' and 'person not there'. You get that from exposed skin,
not by sensing tiny temperature differences on the surface of an
insulated jacket.

--Winston