Thanks for advice, kc1di: I saved all my data in case and tried the synaptic>mint-meta-kde route. All went well until an error message:
E: /var/cache/apt/archives/mint-info-kde_2012.05.02_amd64.deb: trying to overwrite '/etc/linuxmint/info', which is also in package mint-info-mate 2012.05.02
and the install stopped
. It hasn't affected my Gnome system at all, but it won't finish the KDE install. I don't want to rename or remove the current /etc/linuxmint/info file to allow the KDE version to install, in case it DOES wreck the Gnome system and I'm left with nothing!
FYI, /etc/linuxmint/info currently reads
DESCRIPTION="Linux Mint 13 Maya"
GRUB_TITLE=Linux Mint 13 MATE 64-bit
Any ideas welcome!