Thank you for the reply. I had read a number of articles on this which put me off trying that option and implied rather clumsy solutions which was why I raised the request.  However, I have now tried it and barring one problem it works and I can boot off either disk.

The problem occurred when I ran grub-install.  It failed at the efibootmgr stage.

Here is the end of the log when installing to a non-raid ESP.

grub-install: info: copying `/boot/grub/x86_64-efi/core.efi' -> `/boot/efi/EFI/ubuntu/grubx64.efi'.
grub-install: info: Registering with EFI: distributor = `ubuntu', path = `\EFI\ubuntu\grubx64.efi', ESP at hostdisk//dev/sda,gpt1.
grub-install: info: executing efibootmgr --version </dev/null >/dev/null.
grub-install: info: executing modprobe -q efivars.
grub-install: info: executing efibootmgr -b 0000 -B.
BootCurrent: 0000
Timeout: 1 seconds
BootOrder: 0005,0006,0002,0003,0004
Boot0002* Hard Drive
Boot0003* CD/DVD Drive
Boot0004* Removable Drive
Boot0005* UEFI: SanDisk Cruzer Edge 1.26
Boot0006* UEFI: ST31500341AS
grub-install: info: executing efibootmgr -c -d /dev/sda -p 1 -w -L ubuntu -l \EFI\ubuntu\grubx64.efi.
BootCurrent: 0000
Timeout: 1 seconds
BootOrder: 0000,0005,0006,0002,0003,0004
Boot0002* Hard Drive
Boot0003* CD/DVD Drive
Boot0004* Removable Drive
Boot0005* UEFI: SanDisk Cruzer Edge 1.26
Boot0006* UEFI: ST31500341AS
Boot0000* ubuntu
Installation finished. No error reported.


And here is the log for a raid1 ESP

grub-install: info: copying `/boot/grub/x86_64-efi/core.efi' -> `/boot/efi/EFI/ubuntu/grubx64.efi'.
grub-install: info: Registering with EFI: distributor = `ubuntu', path = `\EFI\ubuntu\grubx64.efi', ESP at mduuid/f1f50fa9a6d7d446dddc9c93a8fc41a3.
grub-install: info: executing efibootmgr --version </dev/null >/dev/null.
grub-install: info: executing modprobe -q efivars.
grub-install: info: executing efibootmgr -c -d.
efibootmgr: option requires an argument -- 'd'
efibootmgr version 0.11.0
usage: efibootmgr [options]
        -a | --active         sets bootnum active
        -A | --inactive       sets bootnum inactive
        -b | --bootnum XXXX   modify BootXXXX (hex)
        -B | --delete-bootnum delete bootnum (hex)
        -c | --create         create new variable bootnum and add to bootorder
        -D | --remove-dups      remove duplicate values from BootOrder
        -d | --disk disk       (defaults to /dev/sda) containing loader
        -e | --edd [1|3|-1]   force EDD 1.0 or 3.0 creation variables, or guess
        -E | --device num      EDD 1.0 device number (defaults to 0x80)
        -g | --gpt            force disk with invalid PMBR to be treated as GPT
        -H | --acpi_hid XXXX  set the ACPI HID (used with -i)
        -i | --iface name     create a netboot entry for the named interface
        -l | --loader name     (defaults to \EFI\redhat\grub.efi)
        -L | --label label     Boot manager display label (defaults to "Linux")
        -n | --bootnext XXXX   set BootNext to XXXX (hex)
        -N | --delete-bootnext delete BootNext
        -o | --bootorder XXXX,YYYY,ZZZZ,...     explicitly set BootOrder (hex)
        -O | --delete-bootorder delete BootOrder
        -p | --part part        (defaults to 1) containing loader
        -q | --quiet            be quiet
        -t | --timeout seconds  set boot manager timeout waiting for user input.
        -T | --delete-timeout   delete Timeout.
        -u | --unicode | --UCS-2  pass extra args as UCS-2 (default is ASCII)
        -U | --acpi_uid XXXX    set the ACPI UID (used with -i)
        -v | --verbose          print additional information
        -V | --version          return version and exit
        -w | --write-signature  write unique sig to MBR if needed
        -@ | --append-binary-args file  append extra args from file (use "-" for stdin)
        -h | --help             show help/usage
Installation finished. No error reported.

It had installed grubx64.efi to the ESP but not set up the efibootmgr entries.  Once I set those up manually everything worked OK.

I assume I should raise this as a bug and will do so shortly unless anyone thinks otherwise.