So I went out and bought ten metres of CAT5 from the computer shop down the road (only cost me £6). Then, down in the cellar, I fed one end of it through a hole in the skirting board next to a radiator pipe (meaning I didn’t have to drill anything) into the wall cavity, and then reached round inside the cupboard and pulled it through. Then I drilled a small hole in the lounge floorboard and passed the CAT5 up through it and plugged it in to the router. There you go. Seven cable clips and you wouldn’t even know it was there. Don’t know why I didn’t do that months ago rather than fighting with unreliable wireless.