I did a little looking around for you, and saw that the only people that were able to fix it, either used a heat gun around the ethernet port area to try to reflow the solder joints, or were able to see some serious warping of the motherboard in that region and alievieated it with spacers against the frame.
Either way £68 is too much for a repair, better off selling it as is, cuz you never know what kind of junk M$ will send as a replacement.
I wouldnt recomemnd opening it, as this is one of those things that may or may not be able to be fixed, so unless you are well skilled at 360 repairs probably would be best to cut your losses while you can.