Progress...

Manually partitioning can yield the required layout.
After defining the device as encrypted it is necessary to go back and unselect it's usage which partman automatically does without notification.
Removing the ext4 filesystem here then permits selection and creation of a VG in in the LVM configuration menu, placing LV's on top.

Issues..

1) The ability to select F2FS as the filesystem when defining a partition's usage is missing.

2) During boot loader stage installation 'grub-install-dummy' fails.

    Fixed by the following (note must be a lower case 'y'):

    chroot /target
    echo 'GRUB_ENABLE_CRYPTODISK=y' >> /etc/default/grub

    # manually install GRUB, although installation from the Menu should now work (untested)
    grub-install /dev/sda
    grub-mkconfig > /boot/grub/grub.cfg
    exit

    Select 'Continue without installing bootloader'

3) Need to edit UEFI vars and manually add EFI/debian/grub64.efi from /dev/sda1

4) EFI boots, GRUB prompts for password to decrypt, and then drops to prompt.

5) I have no idea how to proceed, assume issue with Grub but could be the console wetware :(