Something is broken in apt/dpkg, how to recover?

[Chris Green]

Something is very broken in the depths of apt/dpkg/synaptic on my
system.

Any attempt at running apt-get results in a segmentation fault and
also any attempt to run synaptic fails - it authenticates then the GUI
appears but before it populates with lists etc. it crashes.

I *suspect* that it's something in the 'update' part of apt that's
broken, if I do "show details" on the crash report from synaptic it
says the executable is /usr/lib/update-notifier/apt-check.py.

How do I fix this?  I can't use synaptic or apt to reinstall anything.

-- 
Chris Green