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I've recently moved into an 80s built house and i have some questions about the wiring i've found.

I was looking at replacing the downstairs switch to the upstairs landing (2 gang, 2 way) for a white switch like for like. At this point id say the switches for this light work as expected and i'd assumed the left hand switch operated the upstairs light (it did) and the right hand switch was for a garage light that was no longer used as the garage had been converted and new lights added and the switches moved inside the new room (I'd also found what i thought was the remnants of that switch in a junction box in the loft of the converted room).

With new switch in hand, i isolated the supply to the switch, checked that i had, then removed the switch from the wall.

The wiring from the switches i found is as follows:
Landing light switch (Left hand switch):
Com: connected to red cable
L1: connected to blue cable
L2: connected to L1 of the right hand switch using single core grey cable

Mystery switch (Right hand switch):
Com: connected to single core grey cable
L1: connected to L2 of the left hand switch
L2 not connected

I thought L2 from the Landing light switch (Left hand switch) would terminate at the switch upstairs for the landing so i don't understand why it would go through the mystery switch first, but to make it worse i can find no combination where the mystery switch does anything. Why someone would do this i have no idea. However if this is the case i have no idea what is connected to the random junction box i found above the switch (albeit on the opposite side of the wall) which i assumed would go to this switch. There is no other electrics even close that would have been part of the pre-conversion build?.

Now i'm well aware i'm probably going to have to contact a competent person. Just wondering why anyone would do this or if someone could help me understand what is happening?
 
Thanks I'm at work at the moment, and i'll upload some later, i could draw it out in the interim? Bugs me not being able to understand things even if i cant fix them.
[automerge]1573734086[/automerge]
It looks something like this. When i get home i'll take pictures of the ceiling rose and both switches
 

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From your first post it is three core so there should be a yellow as it also has red and blue. Does the two way switching still function.
 
Yes the 2 way still functions that what baffles me most, i thought the right hand mystery switch would interrupt the operation but it has no bearing.

I've written out a chart to check of all combinations to see what happens
 
So the cable into the switch is 3 core with the blue and mystery cable coming out of l2. Which is why the switch still works regardless of the state of the mystery switch. At the weekend ill switch the 2 gang switch for a single with the mystery cable left behine the plate.
 
I think that's probably something i'll find more as i delve deeper into the house, but thank you for your help. I'm sure it wont be the last time i call on it. Much appreciated. All removed and made safe, was hoping to use this switch to control the living room lights but i'll get someone in to look at that, and lots of other things i've found.
 

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