There must be something better than this ...

[#1]

@oldjim
Just disconnected 3&4 from the master socket ........ big connection speed boost, thank you
Speeds yesterday morning (Sunday) were test socket= 5984: elsewhere= 3136
Now, at this extension socket, connection speed = 5600
Disconnecting the ring wire has not changed Line Attenuation or Noise Margin.  They are 43 & 12, same as everywhere on Sunday
I will check at the test point when I get an opportunity and post again 

[#2]

Glad to hear you got a speed boost from fiddling with the wiring.
I'd be even more interested to see the test socket stats now.

[#3]

@MAND
OK here is the summary of the router stats
The connection speed at the TEST socket used to be higher than elsewhere
The bell wire was disconnected from the master socket
The connection speed elsewhere was much improved
Here are this mornings router stats
13 05 2010 08:00
Without the ADSL filter
TEST socket 5824 43 12 448 13 23
Without the ADSL filter
FRONT socket 5824 44 12 448 13 23
13 05 2010 08:30
With ADSL filter
Extension 5536 42 12 448 13 21

The Line Attenuation and noise Margin figures are pretty much as they have always been when I have tested.
Please let me know if this can be improved in any way

[#4]

Your target sync speed is 6000-6500 for your attenuation. Probably because of your problems your target noise margin has been increased to 12db which will have lowered your sync speed. If the BT DLM system sees the line remain totally stable with few errors being logged it will lower your target noise margin by 3db and you will then see increased speed - unfortunately this can take a few weeks (unless the training of your line is restarted).

[#5]

That shows that you've sorted the internal wiring issues (same speed from test socket as from extension).
I would return the setup to how you want it to be, and see whether it stays stable for a few days.
If it does I'll get the SNR reset for you, which should give you a little more speed.

[#6]

@MAND
I am intrigued by these "internal wiring issues".  The house was built like this 8 years ago.  Nothing has changed wiring-wise or equipment-wise for years.  The real slow performance is fairly recent.
It is true that disconnecting the ring wire in the master socket (and I assume most houses in the UK still have a ring wire connected ....) did markedly improve the connection speed.  However, it did not change the download speed.
Since then, on 18th May, Plusnet advised me that “Your speed profile has now increased to 4500kbps. Please could you monitor the connection and let us know if there are any further drops.” 
The download speed since then has been good.  So what does this mean ?  What was it set to before and why ?  And for how long ?  I really would like to know how this situation arises.
Since 18th May, the download speed has trebled ! 
2nd May    232  (must have been a particularly bad day - but it was up & down at this time)
10th May  1360 after ring wire disconnected
18th May  3936 after "speed profile" increased
23rd May  4008 (today, sunday morning)

[#7]

The internal wiring from the master socket wasn't put in by BT but by an electrician.  I'm in a new flat (1 year old) and the extensions didn't work at all, turned out the sparky hadn't bothered to connect them correctly.  3 and 5 on Master was connected to 5 and 3 on extensions for instance.  Found out later they were not supposed to have been connected at all, but oh well, saved me a fortune!
Basically, BT only wire you up from the exchange to the master socket, anything from the master socket to anywhere else in your home isn't their's.

[#8]

Quote from: irritablegit

I am intrigued by these "internal wiring issues".  The house was built like this 8 years ago.  Nothing has changed wiring-wise or equipment-wise for years.  The real slow performance is fairly recent. It is true that disconnecting the ring wire in the master socket (and I assume most houses in the UK still have a ring wire connected ....) did markedly improve the connection speed.  However, it did not change the download speed.

It's the removal of the ring wire that seems to have sorted things here, the sync speed is the important one when testing these things (changes in download speed follow but are never immediate).

Quote from: irritablegit

Since then, on 18th May, Plusnet advised me that “Your speed profile has now increased to 4500kbps. Please could you monitor the connection and let us know if there are any further drops.”  The download speed since then has been good.  So what does this mean ?  What was it set to before and why ?  And for how long ?  I really would like to know how this situation arises.

This is the IP profile, which controls the download speed you can achieve, and is dependent on your sync speed. There's more about that here.
The IP profile has recently fluctuated:
24th April - 2000
28th April - 250
2nd May - 2000
4th May - 2500
18th May - 1000
18th May - 4500

[#9]

Plusnet have talked about  "internal wiring issues" etc and "The IP profile has recently fluctuated"  whereas it transpires that the day the IP profile received a boost (the connection speed and the download speed) was the day I was suddenly put onto ADSL2+.
I had known the change to ADSL2+ was coming at some time within weeks, but I understood we were going to be advised when it was really going to take place so we could expect the unexpected ... and re-boot the router. 
So, after all the waffle, I now believe that my weeks of hassle were down to Plusnet switching over to the 21CN network.
The final action was that Plusnet phoned me and offered to send me a Thomson router because they had discovered that some Netgear routers were not happy with the ADSL2+.  It was in this phone call that I discovered that I had been moved onto ADSL2+ on 18th May ........ the day things suddenly improved speedwise.
Since that date, and using the Thomson router, things have worked much better.

[#10]

I honestly don't believe that a switch to ADSL2+ was the cause of the problem (your profile had fluctuated for some time before that).
If you'd seen issues after moving I wouldn't have been surprised (as ADSL2+ uses different frequencies it can expose issues on frailer lines), but the opposite seems to be the case in your situation (things have improved a lot since the changeover).