Thanks for the comprehensive reply!
Sadly I can't remember the connection speeds with the BT router. I did notice that things seem to get quicker when we moved to Sky though -- I was surprised how quickly a BitTorrent came down the line, for example.
That said, none of us in the household download much, and we'd be perfectly happy to sacrifice speed for a stable connection.
The BT box is old (early 90s logo) with a full rather than half plate. I'm not even sure if that's the main one or an extension, because there's another (unused) socket in the hallway near the front door. Could be worth a try in that socket, I guess!
Edit: The router is now in the hallway socket, and the attenuation/noise margin aren't much better than they were, and the speed is now 7000Kbps.
ADSL Link Downstream Upstream
Connection Speed 7006 kbps 605 kbps
Line Attenuation 37.0 dB 22.2 dB
Noise Margin 5.9 dB 10.1 dB
Edit 2: No matter what I do it seems we're now limited to 7000KBps, so it looks like you're right -- the Sky engineers are limiting the speed.
Edited by johnnny (Tue 04-Dec-12 13:51:53)