Hi,
I want to fit a stand alone motion sensor in my bathroom ceiling to swich on my
LED 230 volt downlighters.
Outside the bathroom there is an existing light switch with a brown wire a blue wire and an earth
How do I wire in the motion sensor.

You can buy motion detection switches. They can be set for a specific on time or switched on and off. I will assume you are in Europe. You will need one that functions on 240. Here is another example, internally switched it appears.
220V IR Infrared Module Sensing Switch Body Sensor Intelligent Light Lamp Motion | eBay