2-way switch using 3-core cable in old style lighting circuit

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I have old style lighting circuits, not sure of the correct name, where each light and each switch is connected back to a central junction box. Does anyone know the correct/standard way of wiring a pair of two way switches for a landing light using 3-core cable with this? All the diagrams I see are newer style circuits connected at a ceiling rose.

What has happened is that I have had the CU replaced recently, but the electrician didn't realise the two way switch had a borrowed neutral connecting the upstairs and downstairs lighting circuits, so every time the landing light was switched on it tripped the RCBOs. He came back and fixed the problem by combining the two circuits on one RCBO.

Since then I have been refurbishing the hallway and run a 3 core cable between the two switches to get rid of the borrowed neutral. The original wiring had T+E between the upstairs and downstairs switch, the live taken from the neighbouring switch downstairs, a single red cable from the upstairs switch to the landing light and a single black cable from the light back to the upstairs lighting junction box ie the live came from the downstairs circuit and the neutral from the upstairs one.
 
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He came back and fixed the problem by combining the two circuits on one RCBO.
That did not fix the problem. It masked it.

Since then I have been refurbishing the hallway and run a 3 core cable between the two switches to get rid of the borrowed neutral. The original wiring had T+E between the upstairs and downstairs switch, the live taken from the neighbouring switch downstairs, a single red cable from the upstairs switch to the landing light and a single black cable from the light back to the upstairs lighting junction box ie the live came from the downstairs circuit and the neutral from the upstairs one.

You need to get a live and switched live to one switch connected to L1 and L2. Then run your 3 core (and earth) cable from L1, L2 and COM to the same terminals on the other switch.
 
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Well to change a consumer unit without some degree of I & T prior to changing it is not something I would expect of someone calling themselves an "Electrician".
This problem should have been spotted prior to any change.
Was it a friend of a friend of a bloke down the pub that did the change for you?
 
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