Hi Halestorm, & thank you for the detailed comments.
Okay Paul, I've read through everything, and it just screams that something is definitely wrong with your wiring. Let me roll through some of the things I looked at that scream trouble:
I'm pleased that you have immediately spotted that something is wrong.
I have presented very similar data to my ISP who has been unable to spot any problems (as per their comments that I displayed in red in my previous message).
Max: Upstream rate = 5878 Kbps, Downstream rate = 31480 Kbps
Path: 0, Upstream rate = 5926 Kbps, Downstream rate = 29804 Kbps
Your max rate versus your path are waaay too tight. You are running at 95% capacity down and 99% up. There is no wiggle room to allow for changing line conditions. This virtually ensures that you will lose sync on a regular basis. I would be more comfortable if someone was running at 80-85% capacity, no more than 90%.
Whenever the modem re-syncs or is rebooted the difference between the max rate & the path rate is more pronounced.
e.g. for the connection in my example data, the max rate started off at 35112K, with SNRM at 6.3 dB.
That equates to a path rate of approx 85% of the max rate.
Over the duration of the connection, the max rate for my connection generally fluctuates a little but gradually tails off, as does SNRM.
Just before the modem re-synced itself, the max rate had dropped to 31552 & SNRM was 3.5 dB. (Those were the single values taken from the modem's GUI).
VDSL Band Status U0 U1 U2 U3 D1 D2 D3
Line Attenuation(dB): 8.3 52.9 64.0 N/A 21.6 63.1 0.1
Signal Attenuation(dB): 14.6 52.3 N/A N/A 21.6 63.1 N/A
SNR Margin(dB): 5.8 6.0 N/A N/A 3.5 3.4 N/A
TX Power(dBm): -4.3 5.8 N/A N/A 10.9 6.5 N/A
In reading the properities of your signal, the attenuation isn't as important to focus on as the signal to noise ratio (SNR) margin. If you look, the SNR upstream between the bands is almost the same, as is the downstream SNR. To have a quality signal where it's not taking too many errors, the SNR upstream should be between >6-8dB and downstream should be >9-11dB. Your upstream would actually be okay, but the downstream is what's killing you - 3.4/3.5 is waay too noisy.
I believe that in the UK, the target SNRM is 6dB for DS for ADSL. I'm not sure what it is for VDSL2, but I have seen many examples where users' DS SNRM is between 20 dB & 30 dB.
These levels are from users who currently have max rates of 100000K or so, but are currently capped at 40000K (until BT raises the capping to 80000K over the next few months or so).
I suppose the quality of the signal (SNRM etc.) is critical in maintaining a high speed, relatively error-free connection.
However, to achieve high speeds in the first place, surely my attenuation levels (especially in the upper frequency band(s)) is currently simply too high at 63 dB or N/A.
On the original 8c profile we had fewer frequency bands (2 No.) & fewer tones available for bit-loading (less than 2000 on 8c - now almost 4000
available from the 17a profile).
You can see the differece between my graphs & the other user's graphs, in that I get absolutely no bits loaded in the higher frequency band.
However I was able to sync at around 35 Mb & download at 32 - 33 Mb for the first few weeks.
That leads me to believe that considering the old cabling from the cabinet, I should really be quite a bit closer than the 1000m that has been quoted from a JDSU unit.
A BT engineer in the thinkbroadband forum has stated that he has seen as high as the full 40Mb over 1000m & as low as 16 Mb. I can only assume the 16 Mb connection included substantial quantities of aluminium cabling.
From what I can gather, all other users with lower speeds saw a substantial DS speed increase when switched to the 17a profile that now has more bands & tones available.
Latest 15 minutes time = 13 min 33 sec
FEC: 12342 0
CRC: 3 0
ES: 1 0
SES: 0 0
UAS: 0 0
LOS: 0 0
LOF: 0 0
Previous 15 minutes time = 15 min 0 sec
FEC: 20392 1
CRC: 16 1
ES: 5 1
SES: 0 0
UAS: 0 0
LOS: 0 0
LOF: 0 0
These are the measurements that report the errors of your VDSL signal. FEC (forward error correction) are corrected errors in the signal. IMO, anything more than 2000-3000 per 15 minutes is where trouble starts, and you taking 12k-20k every 15 minutes is definitely excessive and indicates a line fault, bad connection or unshielded/untwisted/unbonded cable that is allowing outside interference or crosstalk from other ADSL & VDSL signals to affect yours. If you were taking more upstream errors, I'd say that AC line interference at the modem would also be a concern, but you're not, so that's not it. CRC errors are uncorrectable errors. What you're taking isn't debilitating, but it would be annoying. ES is errored seconds, SES is severely errored seconds. That's the time of the 15 minute period that your line was taking uncorrectable errors or a crapton (yes, it is a technical term 
) of uncorrectable errors. If one was to have measurable ES/SES on a daily basis, that is a cause of concern and indicates a definite fault. The other metrics UAS (unavailable seconds) would be the time spent out of sync, LOS (loss of sync) and LOF (loss of frame) indicate the number of times you lost connection.
Latest 1 day time = 11 hours 58 min 33 sec
FEC: 1883677 9304
CRC: 9575 43
ES: 152 43
SES: 41 0
UAS: 0 0
LOS: 0 0
LOF: 0 0
Previous 1 day time = 24 hours 0 sec
FEC: 1971373 166
CRC: 5037 110
ES: 227 88
SES: 15 0
UAS: 0 0
LOS: 0 0
LOF: 0 0
For you to be taking this many errors in a single day, and somewhat consistently, to me narrows it down to a cable fault rather than an intermittent source of outside interference.
From a layman's perspective I also tend to agreee with that suggestion.
I do get a few spikes of really high error counts in one hit, but none of them coincide with any electrical eqipment being switched on/off in my home.
I live in a semi-detached house & my immediate neighbour is away quite a bit. I still get the errors while she is away & the spikes are not really at times when her heating could be switching on/off etc.
Now, to elaborate on what a good cable path is for VDSL versus a bad one:
Shielding: Shielding of the cable feeding the neighborhood helps keep out interference in the bands that VDSL operates in - AC line noise, AM broadcast, amateur radio, etc. If your neighborhood has unshielded cable, this greatly increases the noise present on your line. Also, where splices in the cable are made, the shielding between them needs to be bonded, so that interference continues to have a place to go. And at every opportunity (say every pole if it's aerial, or every pedestal where cable may be buried) these bonds should be grounded to dump the interference off of the cable. Aerial poles should each have a ground wire running down the pole to the ground. There's nothing you can do about this, really, except you CAN walk down your cable and look at the poles and make sure the ground wires aren't broken. Buried cable splices are usually grounded in the pedestals where the splices are brought to the service to be accessible to technicians. (Of course, telco terminals are their property, and opening one when you're not authorized by the company is probably a crime of some sort.)
I'm not 100% sure of the cable route as some is overhead & some is underground.
I have walked the 2 likely routes previously, but have not noticed anything untoward.
Twisting: Wire feeding VDSL, when it's not shielded, needs to be twisted cat-5 style. This includes everything from the cabinet to your modem. Common places where wires need to have a twist are in the cabinet (the cables feeding from the fiber cards, and where the cables enter the ground/air before their shields are bonded), at splice points, where the company's cable interfaces with yours at your home (demarcation point), and the wire run from that interface to your modem. The wire in your house should be cat-5 or cat-6, and using shielded cat-5 or cat-6 would help immensely, providing that the shield is bonded to the ground in your demarc. Using a shielded cable but not bonding the shield would not give the interference picked up inside your home anywhere to go and could cause trouble (most likely the problem you had when you used that shielded cat-6). The cord from your service jack to your modem should also be cat-5. Using a flat telephone cord or nontwisted cat-3 wire just lets interference walk right on in.
The shielded, but unearthed cable could have been causing a problem (others had also suggested it should be earth bonded), although it was all fine right up to losing my services for 4 days.
The latest engineer replaced the external drop wire from 2 poles away from my home, right into the master socket (which he also replaced), next to my desktop PC.
the modem is plugged directly into the VDSL socket at this master socket using the BT supplied patch lead, & the shielded cable is now simply used for a telephone extension, hard wired into the removable face plate at the master socket.
So, the problem could have been either the external drop wire or the shileded cable (or even both?).
I have disconnected everything apart from the modem for a week at a time now & then, but I see no differences in error counts, tailing off SNRM rates etc.
Termination: Poorly terminated wire just causes problems anywhere. If your service is running out of an old school telephone jack with screw terminals inside, it could be fine, it could also cause trouble. I personally prefer using punch down style jacks, that way the service cord can maintain its twist all the way to the point of termination, and there's zero chance of a wire being broken on a screw terminal. Also, nicks on a cat-5 from it being stripped, old bad splices in telco terminals where the insulation on the wire is hardening and breaking apart, bad splicers themselves (i.e. wires twisted together with electrical tape versus using a gel-filled splicer) can introduce trouble.
None of the negatives from that section now apply.
AC Interference: This can happen in your home itself. If you have any ungrounded appliances in your house, or if your house doesn't have a good common electrical ground; again, interference has no place to go. Someone who was having trouble with this would have success with an electrical noise isolator plugged inline with the modem's power supply, and/or having their electrical wiring repaired to be properly grounded.
A lot of the house wiring is either new or less than 8 years old, & it has all been tested by an electrician under Building Regulations Part P.
One engineer did wave some sort of a gadget around the power points near the modem to check for AC interference, & gave it the all-clear.
This is hardly inclusive, but these items are the biggest causes of trouble that I've encountered. Of course, without physically being able to inspect all of these elements, I can't tell you what's causing your trouble. 
BUT... anything in your home, you CAN look at and see for yourself if the signal path is pure. 
I THINK that I have eliminated everything within my home that could cause issues, & can only assume that the "fault" is somewhere externally, between the cabinet & my home.
If I had never achieved the higher speeds & apparently stable connection, I would simply accept that what I am achieving now is much better than the 1 Mb I achieved over ADSL.
The copper distance right from the exchange is reported as 5283 metres, which looks about right.
I can only assume that whatever work was done to reinstate my services back in July introduced the "fault" that is now restricting my connection's speed & still leaves me with some connection instability.
If only I had records of my connection's stats from when it was working very well, I might even be able to prove it.
All I really have is the attached speed test.
It may just be coincidence, but now the weather is much cooler than it was between July & September, my connection does appear to be a little more stable.
Can temperature have much of an effect on old external cabling (some of it being possibly 40 to 50 years old)?
I am sure I can't be the only user experiencing these issues, but in the main, any forum posts I have read suggest that users are generally very satisfied with their FTTC connections, managing to stay connected for hundreds of hours at a time.
One final matter, not really discussed previously, is power levels.
I have very recently read that BT can remotely adjust VDSL2 power levels at each cabinet, to avoid the VDSL2 signal swamping ADSL signals that are already quite weak, having travelled a few thousand metres from the exchange over old, possibly partially corroded copper cabling.
I just wonder if BT had to turn down the VDSL2 power levels at my cabinet in order to avoid cross-talk issues after my first month's connection.
The ADSL copper cabling would probably be around 4300 metres to 4500 metres in length by the time it reached the cabinet.
Thanks again for commenting on my issue(s) with a fresh pair of eyes & I fully accept & appreciate the comments in your disclaimer.
Paul.
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