@licquorice That's great - thank you. I've exported a CSV file with the filter set to WAN and I can see the "DSL Link Down" events, which helpfully include the duration.
Thanks again to everyone who commented and offered advice. I've extracted the events for the DSL link going down and coming back, for the failover sessions starting up and being closed when the connection is back and for when the hub is properly connected to the WAN.
From Sun 4/6 to Sun 11/6 inclusive, the DSL link went down 21 times!
I'm starting an online chat with BT technical support to raise a fault. Watch this space...
BT ran a line test and detected an external fault. OpenReach engineer booked for tomorrow morning.
🤞🤞🤞