Hi, Davide-

This may look like a small thing, but from this manual about deb source files here:
[manpages.debian.org]
... I wonder if you might need to remove the slash at the end of your ".....r-project.org/bin/linux/ubuntu/" string in the /etc/apt/sources.list file? I don't have that in mine, and it seems to suggest the blocks following it are treated differently (it might not matter; I'm a bit far from my Linux machine to be able to try it myself).

After that change, you could re-remove R again, and then do the "update" command, and then try to re-install R again. See if that gets you the modern version number?

--pt