47 minutes ago, Muttsnutts said:

You Deffo have to remove the small pcb screw to fit it to a wall. 

 

Installation Instructions
1. Open the housing by removing the front cover. To do so, press the tab located on top of the detector.
2. Remove the PCB by unscrewing the PCB screw from the rear of the protective plastic casing. Note: Do not touch the face of the PYRO sensor.

agreed, normans point is the lid fixing screw (not the pcb screw) will be against the ceiling unless its upside down?

26 minutes ago, james.wilson said:

agreed, normans point is the lid fixing screw (not the pcb screw) will be against the ceiling unless its upside down?

No there is no lid fixing screw ! There is a special tool to release cover ifs against celing, like one in pic ....

The detector in the photo shows it the right way, but that casing is designed to mount the other way up, there will have been a small securing screw near the tab, hence the hole. 

44 minutes ago, norman said:

The detector in the photo shows it the right way, but that casing is designed to mount the other way up, there will have been a small securing screw near the tab, hence the hole. 

Hence the tool for mounting up against ceiling, and screw when lower down if you need it ..........

is that an anti tamper idea or just a designer forgetting about ceilings on a wall mount?

I'd guess aesthetics, smooth lines on the bottom of the PIR.

In reality some numpty engineer turns up and puts screwdriver rash all over it.