I have installed a motion sensor in the kitchen, replacing the original light switch. I have a ceiling light fixture, which has six two pin Halogen GU 10 bulbs fitted and I replaced these with six LED bulbs. They worked fine until after a few minutes when they began to flash rapidly. Is there anything I can add to the light fitting to stop this happening or do I put up with the original Halogen bulbs? The Halogen bulbs are 40 watts each but the LED ones are only 3 watts each. Thanks!
Not all motion sensors are LED-friendly. More than likely, that's your culprit.
Reinstall the manual switch and see if they work.
With LED lights any dimmer (possibly the motion sensor) has to be on the load side (the lamp) has to be on the power source side.