I have had a thought:
I could plug two powerline adapters into the same double socket, connect up two computers, one to each adapter, and try a file transfer that way. There should only be a few centimeters of copper cable involved in the powerline side so I should theoretically see an improvement in speed.
If I do, I can conclude that the restriction is caused by something to do with my wiring I think.
Too late to be faffing about now but I'll try this tomorrow.