As you suggested I started in /var and was soon able to find that it was the dump folder that was full with vma-zst files. I've removed them and all is good again.
At least now I know what to keep an eye on to stop it happening again.
Thanks for your help :)
I'm afraid I'm not very familiar Linux.
Is this where I should be running this command to start with? I've tried it and as you can see it just sits there doing nothing :-(
When clicking on shell on one of my nodes I am getting the error 'Error 500: closing file '/var/tmp/pve-reserved-ports.tmp.1312' failed - No space left on device' and it doesn't open the shell.
I can see that it is /dev/mapper/pve-root that is full but I have no idea why it is full or how to...