I did as you suggested.
To start with, there were 5 broken packages, which I fixed with synaptic.
Next I made sure that udev and udisks2 were installed and up to date. (The latter wasn't, although I had previously refreshed, upgraded safely and ungraded).
Now I am told: No object for D-Bus when I select an external (to the system) partition with pcmanfm.
(I also installed udev-discover and on running it got):
/usr/bin/udev-discover:25: PyGIWarning: Gtk was imported without specifying a version first. Use gi.require_version('Gtk', '3.0') before import to ensure that the right version gets loaded.
from gi.repository import Gtk, Gdk
Traceback (most recent call last):
File "/usr/bin/udev-discover", line 29, in <module>
from udevdiscover import DeviceFinder, get_subsystems
File "/usr/lib/python2.7/dist-packages/udevdiscover/__init__.py", line 23, in <module>
from devicefinder import DeviceFinder, get_subsystems
File "/usr/lib/python2.7/dist-packages/udevdiscover/devicefinder.py", line 26, in <module>
ImportError: could not import gobject (could not find _PyGObject_API object)
This appears to be a systemd issue. (My installation was pre-systemd)
I am going to reboot and see what happens.