I'm in agreement that the best course of action is to continue pushing your letting agency to fix their wiring.
However, since we're on the topic, as a piece of anecdotal evidence, I've had a great experience using PowerLine adapters extensively in lots of houses I/my family have lived in (rented and otherwise). To start with, I've had no problem using them in an apartment building - I think because most apartments (mine included) tend to include a set of contact breakers per flat and wire them all fairly individually. Secondly - whilst your milage may vary, I've never had any problem using them across different ring-mains. My family home has Upstairs Outlets, Downstairs Outlets, and Downstairs (Extension) Outlets all wired separately on different contact breakers and I've got PowerLine adapters on all three which have been working together fast and reliably for over two years. Personally I've had nothing but good experiences with modern 500Mb/s highly-rated (on Amazon) PowerLine adapters.
