Thanks guys.  Today the car stopped again, but I had my trusty
voltmeter with me.  I measured the voltage on the coil: ignition on -
nothing.  Cranking - 12v to chassis on both red and blue wires, of
course no voltage across the coil.  I tried earthing the coil body/
amplifier heatsink - no change.  I don't expect either coil connection
to be grounded but the fault must be in whatever it is that drives the
coil amplifier.  Time, I thought, to read the fault codes: Socket A
pins 2 and 3 give 1-1-1, socket B pin 5 gives 1-1-1.  None of the
other pins give any flashes.  I hope this means something to
someone... the temp. gauge was doing the same thing: switch on, the
needle goes to halfway, pauses for  second, then goes right up, pauses
for a few seconds then falls to zero.  After an hour and a half, as I
was attaching the tow rope, my wife tried the key and she fired up,
drove home with no problems.  I didn't mention it before, but the
central locking gives trouble in damp weather, especially the
tailgate, and the remote often will not lock the doors although it
will unlock them after I have locked them manually.  Could this be an
immobiliser fault?  Where is the immobilser located, and can I disable
it?