Here's a fuller statement of my problem.
I've just bought a new Dell XPS 8960 with 1TB nvram and 64 GB memory.
Dell website for this model offers a BIOS update marked as 'crucial'
and I verified and installed this (XPS8960_2.50_x64.exe) successfully.
Then I verified and installed ubuntu-24.04-desktop-amd64.iso. This
seems to install successfully. The installation ends by offering a
button marked 'Restart'. When I press this button I get a screen
telling me to remove the installation media and press 'ENTER'. When I
take out the USB stick and press 'ENTER' the machine boots up into
Ubuntu Mate which seems fine, but from previous attempts to install I
know that if I exit this the system will not boot again.
So at this point I downloaded, verified and ran boot-repair and
generated this newer boot-repair report Ubuntu Pastebin. I did not do the offered boot repair because I thought it better to ask for some expert advice first. I have not edited or shortened the report in any way because I don't think I have the expertise to do this helpfully.
I'm mystified because the new Ubuntu seems to be there but the machine
can't seem to find it to boot to. At this point I'm starting to worry that I have an expensive boat anchor.