Eurika!
I think today's engineer has found the problem. Engineer this morning understood and could see the large fluctuations in my upstream whilst the phone was in use... and said he was off to change the d-side. A few mins later he called me to 'come look at this' as he noticed that the d-side to the BT66 was fine. He showed me the tests on his JDSU and sure enough the line was rock steady even with the phone in use and phone ringing.
So he comes back inside, and plugs his JDSU again from the test socket... line starts fluctuating and upstream dipping, along with the max sync dropping. So its not the fttc i-plate. Removes the NTE completely and tests from the bare wire. Line is fine.
Repeats the test, puts the NTE back on, trys the test socket.. line plays up.
Gets a brand new socket, installs it... and line is perfectly fine.
Ive no idea where the NTE came from, but according to todays engineer its not a new one, as he can tell by the badging. It's possibly the same one thats been moved from the old NTE location. I think the engineer was a bit surprised as he said its not an obvious failure point and not something they normally look for. I repeat this is not the face plate, nor the i-plate that failed, but the back part of the NTE
For the first time in several weeks my SNRm doesn't budge when the phone is in use nor when the phone rings. I really really hope that this is what it was.