Someone (who shall be nameless) impaled our 406 on the towbar of a
vehicle which stopped unexpectedly. The damage seemed superficial but
turned out to include a busted fan blade (and goodness knows what
else). My question is: can the damaged fan motor be safely disabled
(until the repairs can be carried out) by disconnecting the motor on
that fan or will this cause problems (through confusing the control
system?)

I have in mind the following post by Bob minchin some years ago (and I
have confirmed that my 406 uses the Bitron controller he describes:

If the 406 is anything like the 405 in this respect, the control of
the
cooling fans is managed by a module known as a Bitron which takes
inputs
from the Aircon controller and a sensor built into the radiator and
then
decides whether to switch the fans on either in series or parallel by
means
of three relays mounted behind the front grill alongside the fans.
With a cold engine and air off, the fans should not turn at all.
switch on the aircon and fans should run in series ie low speed.
If the engine gets hot when aircon is on or off, the fans run in
parallel ie
fast.
If one fan runs and the other does not when in series, it just means
one
need freeing up /oiling.
If one does not run when they are meant to be fast, suspect a relay or
motor.