Cracked it.
For others that end up here ... Grub was failing to decrypt but not notifying.
Converted slot key from default of argon2i to pbkdf2 as per the following document.

https://cryptsetup-team.pages.debian.net/cryptsetup/encrypted-boot.html

Thank you to you Ralph for bouncing ideas, and also thanks to the cryptsetup-team author.
Now all I have to do is make it work the way *I* would like without breaking it again!!!

NB. Any volunteers to add these steps to the Devuan installer and then push it upstream ?