Andr3w,
As I see it if you want Sky then you have three practical options:
1) Stick with the single cable and use a Sky+HD box in "Single Feed Mode". This means the box will play/pause/rewind and also record, but only record the live channel i.e. you won't have the "watch one while recording another" feature.
Single Feed Mode is designed exactly for situations like yours. It prevents the box swapping to the unconnected second LNB input which is what makes recordings fail when only one cable is connected.
2) Get an ordinary Sky box with a single LNB input and hook up a DVD recorder. This'll work but it isn't very elegant.
3) Get permission to add a Stacker/Destacker as others have suggested.
This will probably involve discussions with the aerial company that wired the block as well as whoever manages the building. The reason is that the satellite and aerial system will be a bit more complex for blocks of flats. There will be a big distribution amp feeding several apartments. If there is a free output on the amp then you might get agreement for them to add the necessary bits.
If this works then you'll have all the features of a regular Sky+/HD install.
The only caveats are that the stacker/destacker needs a really strong signal and very good cable from the amp to the apartment and inside too. With a communal install that is normally okay though. The second caveat is you'll probably have to pay for the bits of gear, the installers time and possibly have to grease a few wheels (

) to get things in motion too.