I've answered a lot of queries recently on another forum about Freeview freezing or some stations being unavailable. I don't think it has been mentioned here yet, did you do the retune on Sept 3rd?
There's a lot of folk who have been caught out by this. They either didn't retune, or (more common) just let the TV do a rescan. It depends on the TV as there are some exceptions, but a rescan often just refreshes where the TV has stored the previous channel memories. That's not quite the same as wiping the slate clean and starting again. If in doubt, have a look if your TV has a First Time Install
or Factory Reset
Be aware, sometimes these will wipe the picture settings (cont, bri, shrp etc) as well. So if you have your TV set 2just so" then write those settings down before you go. Sods Law says if you write them down then they will be there after. It's when you chance it that you get bit in the bum
Re the wall socket. If you're on a communal system then I'd expect Sat 1 and possibly Sat 2 to both be fed from the satellite dish. There's some electronics used in communal system that allows multiple units to share satellite signals in a way that's not possible on domestic systems. I'd expect the socket labelled TV (below Sat1) to just be Freeview. That leaves FM/DAB which is the socket below Sat2.
The last thing I'd expect to see is a power supply for a masthead amplifier. It doesn't make sense because all the cabling would normally go back to a hub location somewhere in the building that acts as a central distribution point. The gear in that hub would be mains powered. So there's no need for a resident to have a masthead amp PSU in their home.
Second, the PSU would only work properly with a connection to a separate aerial, not the communal one.
Any decently set up communal system should give plenty of signal for the TV. They're in a different league to the cowboy-installed domestic aerials, so the installers tend to be a cut above too. I'd be tempted to disconnect the PSU and brown lead completely and just run from the aerial socket to the TV. I would then use the TVs signal strength and quality meter, and expect to see 80-100% on quality, and 50-80% on strength.
The white aerial cable is one of those crappy moulded jobs. They're quite poor at keeping out interference. They also drop a bit more signal level than a decent coax. The design isn't good either. The plug end connections can be broken internally but the plastic sheath hides that, so the cable looks okay but doesn't work as it should. I'd consider buying some decent WF100 coax, some plugs and making up a lead. Or get one complete here http://www.satcure.co.uk/accs/WF100_cable.htm