wiring for bathroom light switch

[peter51]

I've read the old threads on this topic but I'm no better off. I have an old house. I've removed and refitted a new base plate but I have 3 reds and an earth. The reds were wired 2 together and one by itself in the old fitting .
The new switch has the earth terminal on the base plate - done.
Do i assume thhat the single red is the common? do i wire the other 2 reds, assuming they are live, together in L1? Together in L2? Or seperately in L1 and L2? Or am I completely wrong?
Help please, the house is getting dark and the heating is off!

 

[Zambezi]

Ideally you would have a tester and you would be able to see which cable is the actual live.
Connect it up either way and it will work - single red to COM and pair of reds to L1 or pair of reds to COM and single red to L1.
PS if the light swicth operates upside down (switch down turns lights off) then move the wires from L1 to L2.