It was the ignition rotor.
So if you think you have spark and after much screwing around and yanking of hair and "thinking" you have spark, ALWAYS try replacing the ignition rotor.

 

I had questions about the results of my dead car solution over on the Jag-Lovers XJ40 forum so I thought I would post my answer here too. I hope this is not against the rules here.

The ignition rotor in the XJ40 has a resistor inside of it that provides the connection between the tip and the center of the rotor. So all of the spark energy has to pass through this resistor and sometimes this resistor decides it has had enough. At that point it totally stops transmitting the spark through the rotor. In my case it was only partially dead so I “sort of” had spark at the plugs but not enough to actually run the car.

Normally there is absolutely NO spark at the plugs but there will still be spark energy being transmitted through the coil wire. It just won’t make it out through the cap to the plugs.

So yes, as others have said here, it is ALWAYS wise to carry a spare ignition rotor in the car. Possibly also a spare crank sensor.

 

Agreed - I highly recommend installing a new CPS in your Jaguar and putting the old, working one in your boot as an emergency spare. The CPS and distributor rotor button are so easy to replace, you'll kick yourself if you become stranded away from home because one of them has suddenly failed and you don't have a spare with you.