I report today with a happy ending.
For the 3rd time, British Telecom sent out an engineer to try and trace an ongoing problem that I had been experiencing since December.
Unlike the previous two engineers, this one, after doing the same checks as the other two, promptly went of to check the telephone exchange that I connect to, and, found a strange fault. Thankfully it only took him a couple hours to find & fix.
As a sideline, my Lag N/A has gone, tho I cannot fully support the probability that the 'fault' was affecting game play as the length of time overlapped, however, I suggest people with a similar situation, especially in the UK, get an engineer out to check the exchange, you never know, it may work.
Regards to the Dev's who tried to help.