Thank you. Sample interval 5 seconds. Not unfortunately plugged into test socket this time.
Noise margin, connection speed, packet loss, latency and jitter all uploaded to:
http://escapetothecountry.info/Broadband/
Plus raw time, c speed and noise margin figures in a text file.
BB provider, Freeola, has last written "With regard to previous engineer visits, these have categorically ruled out the possibility of this being a broadband issue, as when tested by the engineer at your premises, no fault was found. Continued testing and monitoring of this problem has also reinforced this via the use of intrusive testing."

Have you tried an alternative hub?

There is something odd about those and teh previous plots ...

NM never changing from 5 or 6 dB ... loss of sync connected possibly connected with change in margin.

Agreed with the hub... This seems an odd one. I'll keep my eyes over this one out of interest but let you get on with advising

Yup. Original Belkin, the Belkin we keep as a spare, a Netgear DG834G and a D-Link DSL-G604T. This morning I had the D-Link plugged direct into the test socket with a laptop cabled to it. Same pattern: DSL carrier up, DSL carrier training, DSL carrier down, DSL carrier up ...

Is anything else connected to the phone line? Burglar alarm? Sky box?

Do you use a plug-in phone extension cable? Have you tried a replacement filter?

Have you bought anything electrical/electronic new or replacement? Any wires going over power supplies? Or neatly tied together?

It behaves like this when any router is plugged directly into the test socket, ie nothing else is connected to the phone line at all. The NTE5 is just the other side of a window frame, so nowhere near fluorescent lights, PSUs, etc.