Ok, out of curiosity, I've done a few more tests.
Firstly, the cable runs : the modem is on a shelf about 80 cm above the NTE/5 with a filtered faceplate. The cables do differ a bit in length, see the table below. Each cable followed the same route, namely straight up from socket to modem, although any excess was left to dangle downwards.
Moving the cables about, or even winding them through a ferrite core whilst the line was synched didn't seem to make any difference to the data - I watched the graph as moved the wire or wound it, then left it a bit.
Cable Length Description
1 203cm "Magic" RJ11 flat untwisted cable that gives me best S/N ratio. 26AWG
2 100cm ADSL Nation Screened Twisted pairs RJ11 cable
3 124cm Home-made RJ11 single twisted pair (from old solid core CAT5 cable)
4 100cm Original flimsy RJ11 flat pair supplied with modem, 28AWG
5 184.5cm RJ11 to BT plug from old 56K modem. (Plugged into NTE/5 test socket)
(Lengths are measured plug tip to plug tip)
View the attached graph in conjuction with the cable descriptions above.
You will see that there isn't a significant synch speed change, but that cable 1 consistently gives the highest upstream S/N ratio. The unlabelled start of the plot was from previous days synch, still connected using cable 1.
I was also interested in the amount of ADSL noise radiated from the different cables. I used a MW radio, held at a distance from the vertical RJ11 cable, and moved it around to determine the distance where I could no longer detect ADSL noise from the background hiss.
cable distance
1 21cm
2 5cm
2 (earthed screen) 1cm
3 5cm
4 10cm
5 omitted test - oops!
Unsurprisingly, the screened cable (no.2) radiated the least noise, which was even further reduced when the screen was earthed at one end. Even at 0cm proximity in contact with the MW radio, the ADSL noise was noticeably muted compared with other cables, and I could instead detect some processor noise from the modem.
Make of that what you will, I know which cable is my favourite, and which sees to give me fairly consistent synch speeds
Ian