At 15:30 on Monday 25/08 Plusnet responded by phone and the question advising me to switch router, filter, plug in to the test socket and run a TAP3 test.
Since then I have switched from a trusty old Netgear DG834G4 to the brain damaged PlusNet Technicolor TG582N (I could not get port forwarding to work, settling for putting my firewall (FreeBSD+IPFW) into the router's DMZ) connected to the test socket with the supplied cable and filter and normally no phone connected. This has improved speed a lot, but still not to Be speeds at their best under similar circumstances (short 100m EO lines, Annex M).
In particular my upload speed is down by 0.5 Mb/s.
I see around a million FEC errors per hour, and there is a slight constant hiss when performing a quiet line test with a corded phone and headset. The hiss is also present when the router is disconnected. I assume that this less than perfect line causes the low Annex M upload sync rate, slightly low download sync rate and interleaving.
With a Windows laptop connected to the router and my firewall disconnected the BT wholesale speed test consistently shows my upload speed as below threshold and invites me to run the tap3 test, but when I do my browser times out, which seems to be a known problem.
I have updated the question with my findings throughout, but Business Support have yet to get back to me.
I am unhappy:
With the time and effort that this is taking.
That Business Support only work 08:00-20:00, whereas I normally rise shortly before starting work at 18:00 and do not have time to call them till rather later. Residential Support do eventually answer the phone but can not do much for Business customers (different systems?).
That Business Support seem rather slow to respond to question updates, and have asked me to perform a test which they know does not work.
That I have signed up for 24 months (barring future price rise get out of jail free cards) for a quite expensive package (Business, Annex M, priority - both these last are around £10 a pop), which is not as good as I expected.
On the other hand, while PlusNet are a BT subsidiary, so far they seem much better than BT in general and BT Business Local in particular.
xdsl info expand=enabled
Physical Layer Statistics:
Modem state: up
Up time (Days hh:mm:ss): 0 days, 20:10:52
xDSL Standard: ITU-T G.992.5
xDSL Annex: Annex M US 56
Channel Mode: Interleaved
Number of reset: 1
Chipset Vendor info (G.994.1): Local Remote
Country code: B500 B500
ID: BDCM TSTC
Specific: 0000 0510
System Vendor info (showtime): Local Remote
Country code: 0F00 0000
ID: TMMB ----
Specific: 3C61 0000
Bearers generic info DS US
Payload rate [Kbps]: 20727 2070
Attenuation [dB]: 4.5 5.0
Margins [dB]: 4.4 7.1
Output power [dBm]: 19.4 12.4
Number of bearers: 1
Bearer 0 DS US
INP [DMT symbols]: 1.07 0.11
Delay [ms]: 5.65 1.60
Depth : 96 0.00
R: 16 0
G.997.1 Statistics (Current):
Line failures Near end
Loss of signal (LOS): 1
Loss of frame (LOF): 9
Loss of power (LPR): 0
Line PM: Near end
Error second (ES): 13
Channel PM: Near end Far end
Code Violation (CV): 2 14
FEC: 21203537 17
ATM data path PM: Near end Far end
HEC violation count (HEC): 1 N/A
TBB speed tests
Date Down TBB Down HTTPx6 Upstream IP Address ISP
Wed 27/08/14 01:20 17.13 Mbps 16.95 Mbps 1.82 Mbps - Plusnet
Fri 01/08/14 08:54 20.90 Mbps 20.37 Mbps 0.91 Mbps - BT
02/01/12 22:09 Mon 19936 Kbps 2352 Kbps - Be