I am planning to create a second extension (just for the Sky box) by putting a screw terminal junction box into the single extension that goes to the "computer room". Is it necessary to ensure that the original extension is not live before starting? It could be a bit of a pain to remove the faceplate where it originates.
No, but remember that the ringer voltage can give you a bit of a jolt, so don't hold two wires at once. ;)
You only need to wire pins 2 and 5 (blue/white and white/blue) on extension wiring when you have all the sockets fitted with microfilters.
What I'd suggest doing is disconnecting the extension wiring whilst you're working on it by removing the lower faceplate of the NTE5 (master socket), and temporarily plugging a phone into the test socket inside.
Whilst you're fiddling with wiring, you'll probably disrupt your ADSL connection anyway.