hi Nigel,
Are your PIR's hard wired back to the light and house.?
I have just upgraded my HC12 PIR links and display.
One remote outside HC TX channel which has LDR's PIR's, Temp/Humm, is received by the house HC, I then add the internal house temperature and real time/date to the RX string, which can be displayed and logged.
I have dumped the Oshonsoft SD card logger and started work on a Arduino pcb which will just log to an SD card.
This longer string is then retransmitted on a different HC channel, so I can receive the string anywhere in the house.
Any PIR activation operates an in house bleeper and an identifying coloured LED.
Eric