Almost certainly inside the "junction box" between the lamp and the PIR into which you wire your cable there will be terminal blocks for your wires.
Three of them will be labelled L N and E
L is where you connect the Live from the commons in the switch.
N is where you connect the Neutral from the FCU
E is the earth
There is almost certainly a fourth terminal ( maybe labelled SL or S ) joining a wire from the PIR ( probably red in colour ) to a wire into the lamp ( probably white ) that is the Switched Live and to this you add the wire from the L1 in the switch.
Ok great. I've now just ordered the lamp so fingers crossed I can get it to do what I want it to do once it turns up. I might wire it all up on the bench first and see if I can get it all to work, ie install my FCU inside the house and before drilling through the wall, wire it up inside. I'll report back with findings!
Thank again.