Thanks for the reply brbo.
Only Safari extensions I use are 1Password and Devonthink. Turned both off and restarted - no change.
No virus checkers software installed - some time ago I took a look at Malwarebytes, but removed it.
Use pretty standard 3rd party apps - Adobe apps, GraphicConverter, Carbon Copy Cloner, DropBox, and the like. No recent app installs that I can remember.
Booted in Safe Mode with no change.
No VPN used. Apple Firewall off.
No problems connecting with the internet - web pages open normally - only saved webarchives created recently and in the past by Safari have this problem. Again my desktop Mac opens the same webarchive files normally.
My guess is that a network setting is incorrect or some system file has been corrupted.
I may be fated for a system reinstall unfortunately.