Originally Posted by Cragrat

looks to me like you have a wirre missing mate is the downstairs switcha 2-gang jobbie

if it is chances are there will be a permanent live coming from the common of the other switch of the downstairs switch and the two cables you have marked as L & N will be the strappers which go into 1 & 2 of both switchs down and upstairs and the wire that goes to the landing light will go to com or c

is the house a few years old as this was common practice to use the live from the downstairs circuit and the neutral from upstairs (now not allowed as all circvuits must be electrically seperate,an easy fix usually by a competent sparks but also by no means anything to lose sleep over)

hope this helps chap

if my presumption about 2-gang switch is wrong let me know