[Solution] By the time I'd finished crafting the question, I'd managed
to solve my problem, but posted the question ans solution anyway asthe
fact that it was brought on by an updated for 'grub*' still has me
wondering what changed.

S.S. Update Grub & NFS stops. I'm
missing something

sudo mount -va produces (on both client machines)
/                        : ignored
none                     : ignored
/media/cdrom0            : ignored
mount.nfs: timeout set for Thu Jul 30 20:52:51 2020
mount.nfs: trying text-based options
'vers=4.2,addr=192.168.aa.bb,clientaddr=192.168.aa.cc' mount.nfs:
mount(2): Connection refused mount.nfs: trying text-based options
'addr=192.168.27.11' mount.nfs: prog 100003, trying vers=3, prot=6
mount.nfs: portmap query retrying: RPC: Program not registered
mount.nfs: prog 100003, trying vers=3, prot=17
mount.nfs: portmap query failed: RPC: Program not registered
mount.nfs: requested NFS version or transport protocol is not supported
mount.nfs: timeout set for Thu Jul 30 20:52:51 2020
mount.nfs: trying text-based options
'vers=4.2,addr=192.168.aa.bb,clientaddr=192.168.aa.cc' mount.nfs:
mount(2): Connection refused mount.nfs: trying text-based options
'addr=192.168.27.11' mount.nfs: prog 100003, trying vers=3, prot=6
mount.nfs: portmap query retrying: RPC: Program not registered
mount.nfs: prog 100003, trying vers=3, prot=17
mount.nfs: portmap query failed: RPC: Program not registered
mount.nfs: requested NFS version or transport protocol is not supported

sudo exportfs output is fine and lists te required system

L.S. I noticed an Debian advisory about 'boot-hole'(?) about a new
vulnerability in grub and decided id better see if there was updated
code, which there was. 5 programs/files related to grub*.

So performed the usual
sudo apt-get update
sudo apt-get upgrade
sudo apt-get dist-upgrade
reboot
on the 5 active Devuan-beowulf boxen and all went fine.

When bringing up s headless-server whose job is mainly related to file
transfer, none of the nfs linkage were showing up on this. It was same
problem on a user workstation,

I've restarted nfs on the data server and check exports and restarted
nfs on both the client machines, but I'm at a loss to work out what is
going wrong.

NFS is controlled by /etc/init,d/nfs-common, which is I understand now
covers both nfs-server ans nfs-client.

By restart, I mean stop then start as historically nfs didn't restart
cleanly.

Have I missed something?
Something at the back of my mind says 'No, you have to run TWO
processes. So I continue web searching (duckduckgo) and there is a
mention of 'showmount' as a diagnstic, which tells me an rpc is missing.

Solution
I needed to start /etc/init.d/nfs-kernel-server start'.
Showmount is now happy and 'mount -va' on the clients solves the
mounting problem and all is as it should be.

The question; is why did the 5 grub updates cause this to happen?
None of the exported files are essential for booting.
Oh well, something for the spare time.