Loss of service at night (Plusnet)

My Broadband Ping
This has happened before, but I've had a few weeks of no noticeable problems. On Sunday a scheduled backup job failed and BQM reports this:
My Broadband Ping
Monday:
My Broadband Ping


I've gone through the technical support route before and they always say the problem is on our premises, but whatever the problem is it seems to be strongly tied to time-of-day.

Do I need to go through the motions of testing things here, engineer visit etc, when to me it seems like some kind of upstream congestion?

Have you checked your router stats whilst these events are going on? It looks like potentially a line fault that is being caused during the hours of darkness (normally caused by radio interference which is worse during the night).

Unfortunately its an office, so I'm not on site during the outages. In the past I've looked at using syslog but I don't know how to filter messages and the router generates a lot of routine messages.

I did look at using "routerstats", it worked but didn't tell me much as I had good service at that point.