This morning's updates fixed everything. I don't know whether it was the xorg i810
intel driver or what, but I am back to having 2 usable computers.
I tried putting i915.modeset=1 on the grub kernel boot line, but that gave me a moveable
mouse pointer, no mouse buttons and no keyboard, and nothing was displayed in the plasma
folder view widget. The system accepted no input whatsoever (not even Alt-SysRq - no
keyboard).
I had to put nomodeset on the kernel boot line and now I am up and running again.
Thanks