So I spent a very pleasant hour and a half with Les, chewing the fat about the issue whilst he waved expensive bits of kit about. And he detected absolutely nothing that might be causing the problem, which didn't appear to be an issue all the time he was here, frustratingly - the network connection had dropped three times in the morning before he came. He did comment that it was odd the way the signal appeared to weaken at the back of the house so dramatically - but, whatever the cause of that, it's not something transmitting locally.
* I had to do it all again on Monday, on account of the hitherto reliable Windows 7 OS going into meltdown on the kitchen laptop. Which absorbed most of my Sunday and is going to be the subject of a later post entirely to vent my annoyance and help me calm down. I don't care if you're interested.