This is just an update.
For the first time, I encountered the problem in the office. I was using VOIP and the connection kept dropping and returning. I tried switching to an ethernet cable but whilst the LEDs on the socket showed that it was live, the laptop didn't automatically switch to it and I didn't have the luxury to experiment so I had to abandon the VOIP call and use a mobile.
Immediately after that and for the rest of the day, the laptop was connected to a port replicator via USB-C and was completely stable.
This morning whilst at home and following a boot from cold, there was no WIFI. I wanted to follow your advice Mark but I of course couldn't get to it without an internet connection so I have printed that off now! For a while it refused to connect via ethernet as well but it later decided the ethernet was OK and it's be fine for the rest of the day.
I'll provide an update when there is more info to impart.
