Ah! Apparently they do a re-sync every fortnight! To do with BT itself, not the Openreach DLM. 😧
That might be it. You'll find loads of posts around with a bit of browsing in the BT ISP Forum here.
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I checked the hub again this morning, and although the event log only starts from the time of the re-sync, I found system uptime on another page, and that's still showing 8+ days... that's back to the time of the power cut we had, so it couldn't have been a full reboot.
Anyway, still connected at 79.146 down, and all seems well.
>>> BTFibre 2 FTTC
adslmax (knowledge is power)
Tue 07-Jul-20 13:41:03
Sometimes SNR at very low at 3dB can caused more CRC and ES errors. Only DLM will collect it every midnight. Hope your line errors is low, if it was too high then DLM might act the next step to move SNR up by 1Mbps each step as a time until there is no errors on the line.