Since installation of my FTTC, the downstream speed has consistently tested at 44 to 45-ish. Ten minutes ago, the same result - 45.51 on the BTW speed test. So I dug out a fairly expensive purpose made Cat 5 modem modem lead I bought a while ago, branded 'Belkin Pro', too see what difference it would make. And...
Downstream dropped by over 3Mbps. Repeated three times, always the same, a bit over 42.
I wondered if DLM might have kicked in as a result of my resyncs, so reconnected again with the cheap-looking flat cable provided by BT, DS tested at 46.57.
Upstream was much the same, 7.5-ish with either cable.
So, consistently, the cheap flat cable is getting significantly faster downloads than the posh 'Belkin Pro' Cat 5 . I would not of course advise anybody else to necessarily expect the same effects, the Cat 5 ought to do better. But it does go to show, IMHO, we can theorise all we like about what is scientifically optimal, but in the real world random factors do often conspire to surprise us as to what actually works best.
I have no means of getting line stats at the moment, so can't investigate in more detail. One possible clue might be that the Belkin cable, although terminated with RJ11s is fully loaded Cat 5 with 4 pairs, thus has redundant wires perhaps adding to the interference pickup? The Cat 5 is also slightly longer at 4 feet vs 1 metre, but it's hard to believe that would make any odds.