I know the question wasn't for me but I know the answer
You must upgrade the switch, this allows in effect both signals to be merged in the loft then sent down one cable (the existing one) and then split back into two signals at the faceplate in your flat so in effect you get two signals for the HD box.
You say that you have access to the switch, are there any spare ports you could borrow a feed from, or anyone not using theirs. You will need to drop a cable down to your flat though.
Is there anywhere on the roof or out the way you could fit a dish, you could just run one feed from it and use the other you already have.
You could even fit an Octo LNB and offer the use for another seven neighbours. That might stop them getting all shirty and informing management