After running the raft of very recent updates to Collabora I found that it would no longer run. After some fruitless digging around online I recalled some (fairly std tweaks) I did when following the original installation guide that I used.
Revisited and repeated as below and presto, all fixed and running well - hopefully of use to others.
By default, coolwsd enables TLS connection. However, it didn’t find a TLS certificate file, hence the start failure. It’s better to disable TLS in coolwsd and terminate TLS at a reverse proxy. The coolwsd configuration file is located at /etc/coolwsd/coolwsd.xml. However, it’s an XML file, which is not easy to read and edit. We can use the coolconfig tool to change configurations.
Run the following command to disable TLS in coolwsd.
sudo coolconfig set ssl.enable false
And enable TLS termination at the reverse proxy.
sudo coolconfig set ssl.termination true
By default, coolwsd only allows known hosts to access its service. To allow Nextcloud to access the service, run the following command to add your Nextcloud hostname to the whitelist.
sudo coolconfig set storage.wopi.host nextcloud.example.com
You can also enable the admin account for coolwsd with the following command. You will need to set a username and password for the admin account.
sudo coolconfig set-admin-password
Restart coolwsd for the changes to take effect.
sudo systemctl restart coolwsd
Now it should be running without errors…
AND IT WAS 
There was a very short (too short) pause in the update process that could have avoided the xml file being overwritten but by the time I had read the text (and actually tried to recall the details and then whether to keep the old or take the new) it steamed ahead. I think a STOP would be more appropriate until you select the option you want.