Thanks for the advice oldfred. I take your point re backups.
I ran the recommended boot repair, ignoring three messages: 'RAID detected ...' , 'EFI detected – please check the options.', and 'You may want to retry after decrypting your partitions ….. '.
Boot repair ran through some stuff to do with mdadm and lvm2, asking me to copy and paste a bunch of sudo commands into a terminal. Included were 'sudo apt-get purge -y –force-yes grub*-common shim-signed linux-signed*' and 'sudo apt-get install -y --force-yes grub-efi linux'.
Finished with no error reported.
Message displayed 'Do not forget to make your BIOS boot on sda2/EFI/ubuntu/grubx64.efi file!'
I do not understand the meaning of this as I can find no means of specifying a filename in the BIOS.
Boot info URL: http://paste.ubuntu.com/5588757/.
On rebooting the grub menu below was displayed:
Testing with Secure Boot disabled:
- Entries 1 & 2 boot into Ubuntu OK.
- Entries 3 & 4 boot into Windows OK.
- Entry 5 boots into the Windows 'Boot Options' screen.
- Entries 6-8 fail to boot with a 'file not found' error message.
Testing with Secure Boot enabled, the machine boots straight to error message:
'Boot failure: a proper digital signature was not found.
One of the files on the selected boot device was rejected by the secure boot feature.'
It's great progress to have a dual-bootable system.
What should I try to get Secure Boot working?
Is it safe to delete the non-working grub menu entries from '/etc/grub.d/25_custom'?