OK. I did a little digging and found this.
https://github.com/ValveSoftware/steam-for-linux/issues/4321
From the gist of this, the steam installer is in the Xenial multiverse repos but does not get listed in either the Software Centre or Synaptic.
However when you run “sudo apt install steam” it will pull down the Steam installer and dependencies. I took the precaution of uninstalling the Steam installer that I pulled from the Valve’s own repo first, and making sure the Steam repo entries were removed from the sources.list file (or just going into Software & Updates in the GUI and removing them that way). I also wiped the .steam folder from /home, which in hindsight was a bit drastic.
Once they are removed and you run “apt-get update” to refreshed the sources.list file. I pulled the Steam installer from Canonical’s repos as mentioned above, I could run then Steam without the “out of date” pop-up. Note that I had backed up my games prior to moving over to 16.04LTS, so just dropped them back in.
