Hello CallumND,
Try this in your terminal:
# go to your root web folder
cd /var/www
# change owner for all your Kirby files and folders: root > www-data (I suppose www-data is your web user)
sudo chown -R www-data:www-data *
# change right on all your Kirby files and folders
sudo find /var/www/ -type d -exec chmod 755 {} +
sudo find /var/www/ -type f -exec chmod 644 {} +
# check the result
ls -la
# you should see something like that
drwxr-xr-x ... www-data www-data ... -> folder
-rw-r--r-- ... www-data www-data ... -> file