My line has been up and down like a whores undercrackers.
This has been repeatedly reported to O2, and their best efforts to try and resolve it, but the problem with intermittent problems is that they are intermittent.

After two weeks of problems, being moved to a 15db Snr, and unholy noises on the line when the broadband is on, I get a broadband engineer.

He monkeys around in the exchange, putting me on a shorter cable, manages to drop my loop loss from 63 to 48.

Then he comes to my house and changes my faceplate to an NTE2000.

Even on a 15db line, I was getting 2mb.  Back down to 6db I get 3.2mb.
Cant wait to get my Netgear back so I can tweak the stats.  I reckon I can get nearly 5 and break the 1mb upload

Yeah.  Contention ratios is essentially how many people are fighting over the bandwidth.

As Broadband is sold on the basis that not everybody will be using 100% of the bandwidth, 100% of the time, they oversell it.  Thats the contention.  20:1, 50:1, 1:1 etc.

http://en.wikipedia.org/wiki/Contention_ratio

The actual max speed you can get though ADSL is ~7.2 Mbps, due to overheads.  Much in the same way that you can get a max of ~22Mbps on ADSL2, not the advertised 24,