...Not our gremlins...

Well, we thought the network gremlins had been chased out, but issues persisted unless I was using wifi in the cabin. If it wasn't the switch, the hub or the cable, then it must be something beyond the end of the ethernet connection...

I don't normally call upon work IT support for domestic issues, but this domestic issue now looked as if it was being initiated by work kit. When I got my new work laptop (November-ish?) I had it plugged in to my own port replicator and all was well. That trusty beast decided it didn't want to handle video any more, so I started using the mini Kensington hub supplied with the laptop plus a USB hub, because there weren't enough USB ports for keyboard, mouse, camera, speakerphone and printer. I swapped out the hub for a different one and it made no difference, so my suspicions fell squarely upon the hub itself. I described the symptoms and my suspicions to IT at work, and got the reply "it might be the Kensington hub - a few of those have been dodgy..." - would have been nice to know that before I replaced two switches and recabled the external ethernet...
Sure enough, work sent me a replacement hub (big beastie, like the one I have in the office which has enough USB ports) and hey presto, the gremlins seem to have disappeared and I could once again enjoy days full of Teams calls without terrible video and audio quality.