Step away people I know this one…………….
Oh, yeah ok probably a knackered MCB as said above. *Quickest way to check would be to put your live for the upstairs lighting in the downstairs lighting MCB and our neutral with the downstairs neutral. I'm putting money on it being wylex, not much mind.
*for the lawyers; obviously you should be competent and supervised by a qualified electrician, making sure the power is off :D