You keep talking about speeds in terms of Bytes ("B"). BB speeds are measured in bits ("b"), Which did you mean?
My apologies. I typed for a visual clarity (not my usual style). But, since we know what we're talking about, we know what we're talking about.
Testwise, I've done all that I've been previously advised to do, to no avail. The cabinets (we've got 2, now, since the old one is still there) are only about 200 metres away and my reported speeds do not vary with time-of-day and the differences in traffic one would expect with that.
So, we're still stuck with the question of why typical speeds should drop overnight, from 0.6Mbps to 0.2Mbps, with no changes on my part?