Well Weaver going right back and re-reading your original post, I would think the easiest means of WiFi is a separate 2.4Ghz channel to the shed from the upstairs window. Think there are quite a few external aerials that can be plugged into a suitable equipped router at the shed end. If a separate router is used at the house end to deal with the different channel required, any problems due to the distance, and error correction would not affect the remaining WiFi set up. Also you have not got giga fast broadband, so unless you are sending files between computers, I can't see the need for fast WiFi. Therefore you have the full 13 channels to play with. So finding a free channel when your neighbour moves in, should still not be too much of a problem I would have thought. If the distance to the shed is too great, as I suggested in my last post, a high gain aerial at the shed would sort the problem. As I previously said, the resulting WiFi congestion that you would be suffering can't be worse than exists at my home. Also, I if my understanding of WiFi is correct, the actual traffic passing over your WiFi would quite low anyway, as there are only the two humans in your home normally, so any channels would barely be actually sending much when in idle mode, so interference between channels would not be a problem...........?