If your up and down circuits are on different fuses you would have a problem. Use twin and earth 1.00mm from your supply (upstairs is probable easier) to the first detector and then three core and earth to link them.
Most domestic telephone systems take a maximun of three as far as i can remember, usually daisy chained. It is referred to as R E N or ringer equivalent number.
I have just had the exact same problem, living near the sea doesn't do them any good and was able to knock the handle(with my hand) and break the plastic part of the lock and open the handle.
If it "fell" out then it sounds as if the friction needs adjusting at the top of the ladder. Always push up the bottom section first and then the top, lower in the order of top then bottom. You should have a pole to reach the ladder from the ground.
Your best bet is to lift floorboards upstairs and see which way the cables run, looking at your pic it would seem that you already have a junction box somewhere in the run. What is the other large single for? Is it live?
If you do take the supply from the socket you will need to "downrate" the cable by running 2.5mm to a fused connector and fitting a 3a fuse then running 1.00 for the rest of the circuit. If you fit a Switched fused connector this can be used as the switch for the light if you can fit it in the...