Looking at your SNR Margin it looks like your Target SNR has been set to 15dB, which is the highest profile... but indicates that in the past the line has been unstable. Pointing to SNR problems and a possible REIN fault.
Yes, I have plenty of sync/problems and disconnections (as you can see from the graph below- starts late April 2006) but I'm pretty sure it's not a REIN fault. The severity of the problem is predictable ... sunny days give a high frequency of issues, while during cool cloudy weather the ADSL tends to normal behaviour.
(https://forum.kitz.co.uk/proxy.php?request=http%3A%2F%2Fimg329.imageshack.us%2Fimg329%2F8485%2Fsynchistnz2.png&hash=baed61b5f204bfd03c02ceb484541792bc48cb79)
If I were you, when the engineer calls.. offer him a cuppa.. tell him all the things that have been mentioned so far.. say that you are concerned about any charges that may incur, but stress that you have tried everything you possibly can yourself to eradicate the cause of the problem.
When I mention that the line problems started around the time my double glazing was fitted (and he therefore has to investigate) won't this cause the visit to become instantly chargeable ? :-\