After abit of help on this one.
I have just fitted an exterior light to the house (wiring is already there but not connected)
Upon exposing the wiring, I expected 3-core cable (Brown, Blue, Earth) but I was presented with 4-core cable (Black, Grey, Brown, Earth)
I connected the lamp fitting as follows;
The switch on the other side is a double, which controls the hallway light and outside light.
The outside light wiring has not been connected to anything on the switch - this is where I'm stuck!
Here is the wiring behind the switch;
The wiring from the outside light fitting is all on the left.