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Bingo. Wasn't sleeved. And had sat for so long I totally forgot about it. As soon as I worked it out I remembered thinking back to myself 6 months ago saying "I need to remember this....".
I just went black to blue and red to brown. Was easily fixed, and thank fuck for the new CU unit we got a few months ago. However, note to self, test everything before putting the whole bedroom back together in case you need to go into the floor again.
Ok, I'll play.
You connected all the similar colours together instead of the correct wiring for a 3 plate where the switch line neutral is effectively a line conductor and should be sleeved to show that.