when you go to sign up with BT (or other ISPs) using your address opting for new line install....what happens?
this will be almost impossible to sort out with sky - I've seen countless similar situations on Sky-Forums and they are pretty much useless when it comes to unregistered lines/addresses etc, they will tell you its up to you to get the address registered as a separate 'flat' (e.g 1a, 1b, 1c, 1d etc) and then come back to them. As far as im aware, Sky check against BT Openreach's database for registered addresses/lines etc so you need to get your 'new' address on that database so that it will work with sky - but how to go about that is a mystery.
you need to find out if the NTE-5 in your flat is the 'registered' one for the old 'house' address, or if each flat had their own wired into a junction box outside in preparation for future use.
the problem is that Openreach deal with all this but they dont deal with Joe Public, they only deal with ISPs, so it makes it difficult to sort these types of issues out.
you will have to pay for a new line install regardless, even if its just activation of the currently installed line, we had too, BT charge the most at £129 - you may want to look at an ISP with cheaper or FREE line installs... Talk Talk was free last I looked.
good luck anyway