Hi there,
This was completely my bad! I’d setup proxytunnel as per the instructions here HowTo: SSH into host device for my proof of concept host server / service. I hadn’t re-run the balena api-key generate proxytunnel command and so I was still using the old credentials in my ~/.ssh/balena-ssh file.
I re-ran the api-key generate command, applied the new key and I’m back in business. Thanks so much for all your help.