I thought you couldn't have them on the roof if they disrupted the skyline? Having them on the back doesn't work if your line of sight to the satellite is at the front of the building.

And while in theory, yes, one dish can work for a block of flats, there are some funnies with that. You can't have Sky+ or Sky HD if you have a communal dish for example.

I do think that people should put them up as unobtrusively as possible and try to share dishes wherever possible. But one problem is that many of the Sky installers are sub-contractors who don't get paid to do a tidy job - just to put up as many dishes as possible. I've seen several who won't take down an old dish but just put up a new one next to it!