In view of that picture of the consumer unit, (a bit different than the description and certificate) I would hazard a guess that you might have what we call a borrowed neutral on the stair landing light circuit.
If I am correct, this will indeed require some rewiring of one of the lighting circuits.
Not 100% sure I agree with this.
you could find the fault, fix it and replace the MCB with a RCBO for all the lighting.
How old is the property ?