DOMESTICS and 2 WAY again

[SUNRAY]

This morning from a neighbour: I've replaced a switch and it doesn't work.
This is how I found it wired and their sketch of how the original was wired:

Ah simple I said out loud... and swapped red and yellow over, in the process I found the yellow wire was clamped on the insulation.
At that point I accepted the COT and watched as their 6yo grandson checked functionality.
To say I got very confused watching him run up and down stairs flicking switches under instruction was an understatement. However Granny said it looked right.

Basically when the landing light was switched on it had to be switched off in the same place.
The multimeter gave some very weird results.

Then I carefully removed the upstairs 2g switch, which is for the landing and bathroom lights, from the wall paper and many layers of paint to find this:

Predictions on a postcard please but please realise the 2G switch has not been touched for several decorations.

 

[AndyPRK]

I guess it never worked correctly
And that’s why they tried changing the switch.

I predict the bathroom light didn’t work unless the other switches weren’t in a certain combination.

What is cOT

 

[DetlefSchmitz]

Cup of tea?

 

[SUNRAY]

I guess it never worked correctly
And that’s why they tried changing the switch.

I predict the bathroom light didn’t work unless the other switches weren’t in a certain combination.

What is cOT

Give the man a cigar, basically the landing light had to be on for the bathroom to work.

Yes cup of tea and the switch got damaged, they have builders in at the moment sorting out water damage and they suspect it got knocked by them.

I must admit my first assumption was the linked L1's were the permanent live but the meter pointed to the red/blue.
After my initial red/yellow flip it looked like this:

The neighbours were well and truly established her before we moved here in1994. I was gobsmacked when they told me it has always been like this and they turned the light on to check the stairs were clear then turned the light off again before climbing or descending the stairs.

Annoyingly it was such a simple change to put it right:

What's even more annoying is they had someone look at it when they first moved in. My identical house has red, white and blue cable for the 2 way circuits so I think theirs was altered between 1968 and when they moved in.