I am hoping to use a cheap PIR to monitor outdoor activity at night, but
the one I have appears to be too sensitive, it doesnt have the usual
adjustments so may try and block some of the lense range.
Anyhow I opened it up and there is a sensor and then an LED `bulb` which
comes on when activated. Is the LED light necessary for it to operate
as I would prefer it didnt light up if someone is snooping around my
back garden at night.

It is a wirless battery operated cheapo whereby the sensor is in the
garden and the receiver is wireless in the house.