Hi Lee,
i would bring your brown, blue and earth from the light into a light switch.
Then I would bring in the supply for the light in to the switch as well.
I would put the brown from the light into the L1 of the switch (with brown sleeving to mark it as a switch).
Then put the brown from your supply into the common side of the switch.
Put the blues (neutral) into a connector block inside the housing of the switch.
If the housing of the switch is a metal one, there should be a screw where you can put the earths into. If its a plastic one, put the earths into a connector block.
Now you can use the switch to turn on the pir light and if you read the instructions, it may state that if you turn on the switch on /off / on once or twice, the light stays on. You may need to switch it off for 20sec after this in order for it to return into pir mode.
REMEMBER though, check the manufacturors instructions, CHECK that you have the correct ampage for the light from wherever your supply is coming from..