Have I misunderstood your switching?When you say "any unused conductors", does that include the black and yellow, or would it be just the blue?
You would use a Hager J804 and connect as it is nowThen come back on here and ask why it does not work.
The 2 core is the switch drop. The 3 core are the strippers to the other switch.
When you have sealed them off in a box and find it does not work connect the lone red (part of 3 core) to one of the other pairs. If the other switch now works upside down move the lone red to the other pair.
There are always live conductors at switches, ceiling roses, JBs etc when the light is off.It's ok but the blue will be live when the light is off.
Why?Option C involves altering the other switch as well.
Either because it does or because that's what SS suggested.Why?
Particularly as your diagram shows no alterations...
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