After an hour on the telephone to Apple support they finally found the problem in a rogue tracking blocker. I have been using an app called "Lockdown" which installs a VPN. The problem was caused by an update to Lockdown which hadn't been adequately tested. "Lockdown" have apologised and they themselves offered the following fixes:
"We recently released a Lockdown update that, for some users, caused their internet to disconnect unexpectedly. We're sincerely sorry for this, and we're writing to you to take responsibility, provide transparency, and most importantly, tell you the simple fix for this issue:
- Open the Lockdown app.
- Turn off Firewall.
- Turn on Firewall.
That should fix the issue. If not, then you'll need to do the following:
- Open the Settings app.
- Tap "VPN".
- Tap "Lockdown Configuration".
- Tap "Delete VPN".
- Open the Lockdown app.
- Tap to activate and re-install the Lockdown Firewall."
I must take my hat off to the person on Apple's 1st line support desk. Once I had made my way through third line support I eventually made it to the top and all credit to that person they diagnosed the problem in less than a minute and it was sorted. In my case it was "delete the VPN".
Others with this problem may not be using "Lockdown" but it might be worthwhile checking the active VPNs in Settings / General / VPN and see if there's anything untoward there.
Best of luck