2 Way? - No way

[Spiked]

I am trying to put a dimmer switch on what I thought was an existing one way switch at the top of my stairs. I have got two cables into the old switch box. One is two core and earth with the red cable going into the old bottom 1 way terminal and the black cable going into the other bottom terminal. The second cable has only one red cable and that is connected to the top Common terminal.
Whilst trying to connect, having switched off the power to the upstair lighting circuit, I find the red cable that was fitted in the 1 way terminal is still live. Might I have two way lighting to the downstairs hall light despite it being fitted with a one way switch?
Also, any help on how to wire up to a new dimmer switched marked ~/, L1 and L2. Thanks

 

[breezer]

since you turned off the upstairs mcb / fuse and it is still live indicates that it is fed from else where i.e downstairs.

You do indeed have two way switching, a switch with 3 terminals (as you said "red cable going into the old bottom 1 way terminal and the black cable going into the other bottom terminal. The second cable has only one red cable and that is connected to the top Common terminal. ") is for 2 way switching the single red wire should go in the top terminal and the two remaining wires (red and black of same cable) should go one each into L1 & L2 (doesn't matter which way round for L1 & L2)

 

[MANDATE]

Hi Spiked!,
Just want to point out for the benefit of other viewers, you cannot have a two way lighting sytem using a one way switch.
As you know a one way switch has only two terminals and a two way switch has three terminals which can be used for either two way or one way lighting