Firstly I want to flag up that you will most certainly need building control to sign off this work unless your Part P certified, however with the usual politics out the way I will assume you are Part P certified, and proceed to answer your question!
So I understand this correctly, you want to change the downstairs light switch from 2 gang to 3 gang, and use the additional switch to control a newly install outside lamp?
Am I right in thinking the new light switch cable is the one on the right hand side of your picture which has white flex?
The last question is which of the twin existing lighting cables are on the downstairs circuit?
My guess looking at the setup is the ring main is coming into the ceiling rose of the light, so you dont have a constant live and natural you can spur from, just a switched live\neutral.
I would find the cable on the downstairs ring main then jumper across it, so you end up with your light being wired to L1 and Com as your would expect, then an additional jumper wire going from the common on the switch for the new light to the common for the supply on the downstairs lighting circuit, finally take L1 the new light is connected to and add another jumper to L1 for the circuit which pairs with the common you have just spurred from.
With the lighting circuit switched off, and all power isolated and checked. If we assume the middle switch is on the downstairs circuit the picture shows you take a jumper from L1 on the outside light switch (left on the picture) and connect it to L1 of the middle switch, this based on the colour coding should give you a neutral\common feed, I would then remove attach the blue wire from the light switch to L1 (and remove the brown wire).
Next we need to do the same for COM, take COM for the outside light and connect it to COM for the middle switch, finally attach the brown (live) from the outside light and fit it in common.
For completeness ensure the earth from the outside light is secure to the back box earth terminal, which by looking at the picture is the top right, and that should ground it to the earth for the other circuits.
Please let me know if you need anything clarified.
Apologies I just realised one of the feeds (middle switch) is twin and earth and not 3 core, so if you follow the instructions the hall way light will come on when ever the outside light will come on.
Unfortunately the only option I can see is to bring in a new feed or swap the wire on the middle switch for a 3 core and earth, which will allow you to have one core as a constant neutral.
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.