THANKS EVERYONE - It was exactly that, an extra brown wire in the ceiling void, there was a rubber grommet that the wires pass through into the light fitting, it was quite a tight fit and the light was flush to the ceiling, so it hadnt come through when I refitted it.
Took the fitting off again this morning, found the wire, refitted it (and glad to know it had to join with the other 2 brown wires, otherwise it might still have been a puzzle!).
We now have working lights and a new porch light!
Thanks again for the help
