In reply to r1ckyfrankl1n:
Slept there twice, most recently a couple of weeks ago. The toilets are indeed warm, and have hand dryers, which are fantastic if your clothes/fingers/toes are frozen. Last time the toilets were already occupied when we arrived, so slept on the benches in the tunnel, perfectly warm if you have a sleeping bag. My understanding is that the staff don't mind if you're there out of necessity, although this last time I had the impression that the other people there were bivvying to get an early start (they'd all vanished by the time I was woken up by the staff arriving).
We never had any trouble apart from being asked to get out of the toilets before the lift opened, but on both occasions it was pretty obvious that we'd got there pretty late and in bad conditions. They might react differently if they saw you arriving on the lift and setting up camp, I don't know.