Unfortunately, my new PC is about as far away from the router as possible, which makes my wireless solution (£5 802.11n USB dongle) ineffective. I was looking into powerline Ethernet to solve this but then it occurred to me that my solar panels may interfere with its signal.
Is this true?
For bonus points, is it possible to get a strong wireless signal through 30m of stairs, walls and doors?
Your solar panels will not affect powerline, only motors and crossing buses on the circuit box are problems. Also, make a direct connection with powerline to the socket -- no surge suppressor in its way.
Consider MOCA if you have coaxial cable in the walls (note that all splitters in line will need to be 2GHz or higher as MOCA uses the bandwidth above the TV signals).
With 2 802.11ac routers (ASUS TR-AC66U) -- one router and one in bridge mode -- you *might* get that far, but they are still quite expensive and really not ready for general use IMO (but also read THIS). I would try powerline or MOCA first, just buy them somewhere that allows returns without penalty in case you have issues with the powerline.