Success!

Thanks to all who have helped me through this I am now running at up to 5mbps (IP setting but apparently capable of 6+).

If anyone is interested, after checking and establishing that there was a phase mismatch caused by one section of the line being longer than the other(?).

Openreach changed out the line from the pole to my house and installed a new main socket.
There was some discussion about whether the line that I was running from the main socket was causing this problem but it apparently made no difference.

The process has been fraught and.to me, opens up many other questions about whether broadband can be defined in the UK as a Utility, as in other countries; not least of which is that I do not expect (e.g.) electricity to arbitrarily just drop my voltage as they see fit without notifying me. Am I wrong to think this? I have an email from Plusnet explaining all this which I intend to put on our village notice board for others who are suffering similarly and would be glad to publish it here if that would be OK?
cheers,
TerryB

In reply to a post by wdg:

If anyone is interested, after checking and establishing that there was a phase mismatch caused by one section of the line being longer than the other(?).

One section of line being longer than the other? How would that cause a phase mismatch?

Or do you mean one leg being longer than the other? Even then it would need to be significant amount and not just a few centimetres.

Well you may have gathered that, in this case, B******t does beat brains? But since I'm no expert who was I to argue - the engineer (who did a good job really) was backed up by his supervisor who came in (I think!) to find out why the job was taking so long.
cheers,
TerryB