I got it installed at last!
Since writing the below, I have discovered that MOST 1803 installations have SYSTEM in all the User-area folders.
I think I must have done something to remove it on both systems I tried, perhaps a security measure included in something.
I had to add SYSTEM with full control to the Downloads folder with the MSI
This is a fairly new Win10 installation so it seems the defaults have changed in 1803.
Am I wrong?
Another MSI app has failed since, on another Win10 1803, with more verbose information.
Perhaps LibreOffice should change to EXE?
Or (at least try to) translate windows errors like 2755 ?