Solution
All thanks due to @tonyswumac for their help above.
There were 2 parts to this question now they are both solved here’s the full solution.
In /etc/systemd/system/loki.service add the following to the [Service] section:
StandardOutput=append:/var/log/loki/loki.log
StandardError=inherit
If you also look at the setting for ExecStart in the same file you should see the where the yaml configuration file lives e.g.
ExecStart=/usr/bin/loki -config.file /etc/loki/config.yml
Edit the yaml file to set the logging level to debug, info etc.
e.g.
server:
log_level: info
Also, you’ll need to make the folder where the logs will live and give it the correct ownerships. Something like this:
sudo mkdir /var/log/loki
sudo chown loki:loki /var/log/loki
Once done, you’ll probably need to reload and retart the service like this:
systemctl daemon-reload
sudo service loki restart