OK, progress has been made.
Following some tips elsewhere I discovered that /dev/nvme0n1p1 is being corrupted.
dosfsck lets me fix it, but I then reboot into Windows to make sure that I can still at least do that and Windows also then fixes it, so on my next boot to the live USB installer there's corruption again.
But I had to stop for something to eat so I left Windows creating a recovery disk which (I think/hope) includes everything (inc Asus drivers) for a re-install, and then later bit the bullet, took the plunge and did a full clean install of Ubuntu 20.04 (wiping the disk completely and enabling boot disk encryption). And that installed fine.
I still have that Windows installer available if I ever need it but for now I'm setting everything up as whole disk Ubuntu with VMs for Windows stuff (as my wife had before on the old laptop).