The reply before was typed in a hurry when I was trying to do umpteen other things at the same time, so sorry for it being a bit short and to-the-point!
Line stats, well the SNR margin will fluctuate, although on a stable line it won't fluctuate very much, just by 1 or 2dB ... and for your highest possible connection speed, it's the downstream margin you're really interested in. For an attenuation of 48 dB I would have expected you to be able to get speeds of up to 3-4Mb or so, maybe more if you were really lucky. As things stand now, like I said above, you would probably be worse off if you went onto a standard ADSL Max product from BT.
I would try the test socket and see what happens. If you can afford to be without your landline for any length of time you could leave the router in there for a while and see by how much it fluctuates. This would eliminate your own extension wiring.
If the stats do differ significantly between your master socket and an extension socket, then take a look at
http://www.kitz.co.uk/adsl/lowSNR.htm which has a few tips on improving your SNR margin (which is what you basically want to do).
Readings from the test socket behind the master will give us more of a clue what's going on with your line, and hopefully it will indicate where to look at for trying to get things better.