1) I presume there's a service charge and possibly rent on your flat? Therefore step 1 is to raise your concerns with the people making a living from your occupancy. Although there's no legal obligation for them to provide a means of receiving TV in the first place, once they have done so then it's up to them to maintain that service
2) Drop a note in through some of your neighbours doors/letterboxes to (a) introduce yourself and (b) ask if they're having problems also
3) Try Freeview; what have you got to lose? All modern TVs have an aerial socket and a Freeview tuner built in, so you don't need to buy a Freeview box just to do a test unless your TV predates digital TV or is some kind of monitor only. Have a look at your remote; if theres a TV/DTV button, or if the onscreen source menu gives you DTV alongside SCART and HDMI options then your TV has Freeview built in. If the wall plate is labelled for TV as well as satellite then give it a go
4) investigate the possibility that trees or some other plant life has grown and blocked the satellite dish's line of sight. The satellites up in space sit at quite a low angle on the horizon. Here's a video of someone using a sat finder app in their iPhone. As you can see, trees block the line of sight to some of them. The sort of gradual loss of signal you've described could be down to a technical reason such as water damage to a cable or a failing component in the dish receptor or somewhere in the distribution system, or it could just as easily be an adjacent tree or even a weed growing on the roof in front of the dish
Youtube video link:
https://www.youtube.com/watch?time_continue=2&v=5AFtcgYRcu4
Note: It's a minor point, but the signals being received by a satellite dish aren't Free
view. That only comes from an aerial. It's called Free
sat when picked up with a non-Sky receiver, and Sky Free-to-View when being accessed from a Sky box.
It's to do with whose Electronic Programme Guide is being picked from the satellite signal. We here know what you meant, but some folk get their knickers in a twist if it's not described correctly. It's also useful to use the correct description when you're describing the issues to an aerial and satellite-dish installer or to people on other forum sites. That way you'll avoid any confusion or the risk of being sent down a blind alley chasing the wrong solutions
Let us know if you need more help.