Thanks Mike S and Leslie for the diode advice.  I did
manage to see another "4" I think right where it was
cracked.  I put in a new one... 1N4004 and no change.
It still doesn't work.  Ah well...  I'll have to pull
it out again and see if this one burned also.

Unfortunately I'm probably not going to worry about
the heater for now.  I can hear the relay click when I
switch it on - and the fan and fuel pump switch on
after a time - and if I cycle it more than once or
twice some fuel dumps out - glad it's diesel and not
gasoline... I was doing most of the checks in the
Bentley and everything was ok, but...

I think I remember another click (from the heater
relay area) depending on the where the knob was
positioned - thermostat kicking on and off?  It was a
few years ago...  Not sure of all of my memories.
Anyway, not hearing anything other than the on/off
click of the relay at full counter-clockwise.  Ah
well...

I don't have a permanent address at the moment,
wandering around Canada in the van figuring out where
I'm going to live next, so it's impossible to order
parts - this is why I'm not going to be working on it
much more.

All the best,
-Neil