The landlord tested all the wiring to see which was actually live, and the one we are using was supposedly the only one in the house. He idd this shortly after I moved in (the house was newly bought and listed when I moved in)
Should I get him round to do another check?
Any live extension wiring has the potential to affect DSL performance, especially if the wiring uses non-twisted pair cable, the pairings on twisted pair cable are wrong or the bell wire is connected. If you move the router to the master socket and disconnect the faceplate to expose the test socket, you will likely isolate all the internal wiring, though you should verify that every extension socket is dead rather than assuming it is.
By testing in the test socket, you can tell whether the issue is in internal wiring (the landlord's responsibility) or external wiring (which is a fault that needs reporting to the voice provider if it affects voice calls, and to EE if it does not).
Having the statistics from the test socket compared to the usual location is the only way to figure out where the issue lies and whose responsibility it is. Without the test socket statistics as a comparison, we're just guessing. I'm not suggesting you start rewiring the place, just exposing the test socket, testing, then carefully replacing the faceplate afterwards. If an internal wiring issue becomes apparent, it's then up to the landlord to get it fixed.
I have a feeling that the net seems to be even more unpredictable when one of two computers connect, both belonging to other housemates. Could something on their end be making the problem worse?
More than likely, the additional load they add to the connection causes errors to happen more often.