The xps did come with windows and I think it was wiped out by the install of 24.04.
The boot-repair fix to install grub improved things. Now when I boot holding down the shift key I get the grub boot menu. But when I select ubuntu on that menu it goes off to 'Loading initial ramdisk' and hangs there. If I select Ubuntu recovery there it loads what looks like a full version, but still won't restart. I'm very puzzled.