richard@richard-iMac:~$ sudo apt-get update
Hit:1 http://s...content-available-to-author-only...u.com/ubuntu xenial-security InRelease
Hit:2 http://p...content-available-to-author-only...d.net/c.falco/mame/ubuntu xenial InRelease
Hit:3 http://r...content-available-to-author-only...d.com/steam precise InRelease
Ign:4 http://dl.google.com/linux/chrome/deb stable InRelease
Hit:5 http://g...content-available-to-author-only...u.com/ubuntu xenial InRelease
Hit:6 http://p...content-available-to-author-only...d.net/ubuntu-mate-dev/welcome/ubuntu xenial InRelease
Hit:7 http://g...content-available-to-author-only...u.com/ubuntu xenial-updates InRelease
Hit:8 http://g...content-available-to-author-only...u.com/ubuntu xenial-backports InRelease
Hit:9 http://dl.google.com/linux/chrome/deb stable Release
Get:10 http://w...content-available-to-author-only...g.uk/debian testing InRelease [3,157 B]
Get:11 https://d...content-available-to-author-only...x.org/virtualbox/debian xenial InRelease [7,883 B]
Ign:10 http://w...content-available-to-author-only...g.uk/debian testing InRelease
Ign:11 https://d...content-available-to-author-only...x.org/virtualbox/debian xenial InRelease
Fetched 11.0 kB in 1s (7,545 B/s)
Reading package lists... Done
W: GPG error: http://w...content-available-to-author-only...g.uk/debian testing InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 26318813399A679F
W: The repository 'http://w...content-available-to-author-only...g.uk/debian testing InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: https://d...content-available-to-author-only...x.org/virtualbox/debian xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A2F683C52980AECF
W: The repository 'https://d...content-available-to-author-only...x.org/virtualbox/debian xenial InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
richard@richard-iMac:~$