Ian,
You must have a diode across the starter contactor to protect the mag/start
switch from arcing.  A coil builds up an electromagnetic field and when the
connection is broken that field will collapse inducing a current.  That
current will arc across the switch as it opens causing pitting on the
contacts.  There is an AD out on ACS mag/start switches which might as well
apply to Bendix as well (it does apply to some serial nos.).  Mag switches
are rebuildable by replacing the contacts and lubricating with Lubrex as
specified in the AD.  Parts available from Cessna.
This flashover is common to all inductive loads that are switched to some
degree.  Obviously, the primary in a mag is a pretty good coil with many
mhos of inductance.  Other smaller coils don't have a secondary (soleniods)
and do not create such a big back EMF.  The diode must allow the current to
flow but will stop the reverse flow.  If it won't work turn it around.
I can't remember the AD number at this late hour but it should be easy to
find in an AD listing on the web.
Walter Dodson