Powerline ethernet adapters are possibly not going to work that well in a flat if other tennants are using them. They also pose a privacy problem. I have seen reports of people using powerline adapters in adjoining properties being able to sniff each others network traffic. If you do use them, you might want to use a brand/model of plug that encrypts netork communication to ensure security/privacy.
Wi-Fi range extenders could help, but using Wi-Fi for the backhaul between your main access point nd the extender reduces available bandwidth for devices attached to the extender. This too will be compounded by Wi-Fi congestion if you can't find an uncontended Wi-Fi channel. Using Wi-Fi analyser on your phone should give a clear picture of all the Wi-Fi networks across all the available channels in your vacinity.
In fact, if you can run Wi-Fi analyser, you might be able to identify a clearer channel on which to run your Wi-Fi network and that may offer up some improvement.
If it's possible I would suggest swaping the external antennas on your Wi-Fi access point for higher gain alterntives, if your flat is all on one level. I have just done the same to give uninterrupted connectivity in the further reaches of our house and so far th experiment seems successful. Higher gain antennas increase horizontal range at the expense of vertical range, so signal strength has dropped slightly on the first floor of the house above the access point, but its about 5db better in the back bedrooms and bathroom further away from the access point.
The second alternative is add a second access point, set up with the same SSID and passphrase. Connect it to your hub/router with an ethernet (cat5e/cat6) cable. Running a cable between the two might not be ideal, if they are in separate rooms. However, if you can justify the cost/effort of chasing the cable and a network socket at each end into the walls, one close to your router and the other close to where you want the new access point to go, then its the best way of resolving this problem.