Thanks NewW2 - I followed Clem's procedure adding a line to sources.list - after correcting typos the command
$ sudo apt-get update
produced output - here are the last 6 lines:
Ign
http://www.linuxmint.com romeo/ Packages
Hit
http://www.linuxmint.com celena/ Packages
Get: 12
http://www.linuxmint.com romeo/ Packages [2249B]
Fetched 251kB in 5s (48.0kB/s)
Reading package lists... Done
[1]+ Done gksu gedit /etc/apt/sources.list
which looks as though Romeo is added.
Then I entered the command:
$ apt install mintbackup
which gave:
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Couldn't find package mintbackup
I've just tried it again with a capital B - what's wrong?