Alex's assessment is accurate. No progress, still fobbing me off. Have said they've replaced all their kit in the exchange, and now trying to convince Openreach to do the same to theirs - they are now saying they believe the issue is in the OR Layer 2 switches and routing to the headend.
I've sent an email to complaints which was ignored, then when I chased it I got a reply from the Fault manager at the tech support address, so it has gone to them and basically been ignored. They said they'd address it after the issue is resolved, or they could ask their manager to escalate.it now. I asked for it to be escalated, and nothing.
Genuinely, I am now telling everyone not to touch Zen with a barge pole. I appreciate that issues happen, but 1. How you deal with it is important, and 2. This seems to be a fundamental problem.
I don't know how to progress this now. Every route I am being ignored.
Edit: Last 6 hours plot attached for interest.