Thanks Guys. It's been sorted. Breezers note about connecting across neutral and then a few beers with a friend looking at the mass of wires has helped fix it!
I have replaced a light and managed to lose track of what goes where (a few red herrings put there for me too).
I have a mains cable in the ceiling hole and 2 other 3 core wires (one of which I beleive is redundant).
The 2nd wire has no live feed until connected to the mains live and then I...