How to change the FQDN on a Lightsail instance?

I’ve spent the last number of hours trying to work out what’s causing the error “hostname: Name or service not known” to be produced when I run the command “hostname -f” on the latest version of Debian 9. When I run “hostname” I get the hostname ok though and I suspect that it is something to do with the fact that lightsail automatically alters the FQDN hosts.debian.tmpl file (I think). How to I change the FQDN on a Lightsail instance successfully?