For months I've been putting off upgrading to Ubuntu 16.04, but now I think is a good time to do so, however I can never manage to get past the following message:

After updating your package information, the essential package
'ubuntu-minimal' could not be located. This may be because you have
no official mirrors listed in your software sources, or because of
excessive load on the mirror you are using. See /etc/apt/sources.list
for the current list of configured software sources.
In the case of an overloaded mirror, you may want to try the upgrade
again later.

I have checked in /etc/apt/sources.list and see no reference to ubuntu-minimal. Any ideas?