1. If you have (or can arrange to have) a switchwire at one end and a live feed at the other, then you would link between the two switches using the T&E in terminals L1 & L2, connecting the others to the common terminals.
2. If you are trying to make a one way switch into two way without rearranging the wiring, you can't. NEVER USE A CPC (earth conductor) for anything other than earth potential.
This not only includes 2-way switching, but also passive infra-red & timer fan wiring, amongst others.
If a cpc IS used as a live conductor, another person cutting into the circuit cable may reasonably assume the green/yellow wire (in flexible cable) or bare copper wire (in cable) is at earth potential, when it is not.
I drew it upside down. The top picure is the downstairs switches & vice versa.
I understand there needs to be a 2 way sw upstairs & the Black wire shouldnt have gone to the N of the light. The hallway was replastered & I'm guessing the switches were removed. Its strange that it had worked before plastering.
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