Further research suggests this might be an issue with particular gfx drivers. causing X to die on attempted logout. In case it's relevant, I run an integrated Intel HD4000 and discrete Nvidia GT650M.
Several threads like this
show that in this case the solution is to add the line
TerminateServer=true
to the [X-*-Core] section of the file /etc/kde4/kdm/kdmrc
in my case this does fix it, though only after a reboot (which I assume triggers the rc to be reread.)
I saw indications that other distros either force this TerminateServer in order to work around possible graphics issues - or they intelligently add it when they detect likely incompatibilities.
It would be superb if - assuming a more elegant fix is the province of the other team(s) and not Debian - at least this distro could be amended so that the out-of-box behaviour isn't so confusing.
Many thanks and any other info needed, let me know.