Right so the issue you have is you dont have a neutral, you just have a switched live.
So the common lets assume is permanent live, the L1\L2 for that switch will be going to the live terminal of the the light (as when the switch is activated it becomes live), so no matter what combination you use you end up making a circuit which results in the switched live becoming live and the light activating.
If you had a neutral, you could spur off any of the the permanent lives, place the live for the bulb in L1, then use a terminal block to connect the common\neutral to the common\neutral (blue cable) of the bulb.
As mentioned earlier if you can get a 3 core + earth in place of the 2 core + e from your downstairs lights, you could have the 3rd core as a permanent neutral.
The other option is to bring in a 2 core + e into the outside light, which gives you your neutral, which can go to the bulb, then user a terminal block to attach to the live, and then have the switched live come back and plug into live for the bulb.
Its a case of finding out where its easiest to get a feed from.
I hope this helps clarify the setup, attached is a basic wiring diagram.
Please let me known if you need anything clarified.