Jack,
From your comment it seems that you are using some form of testing probe, and not a multimeter in voltage mode for your investigation. I don’t think that this will be very fruitful as it may not register for voltages significantly less than 12v. Please measure the voltage at the + terminal of the coil (a) when the ignition is turned on, but before you try to crank it (b) when the engine is cranking but not firing, and (c) when the engine is running.
With regards to your measurements at the relay when the engine is idling: the C2 terminal (probably the one in the center of your relay - please identify this relay for us by the 5 digit number stamped on the side such as 3xxxx, not the 22RA family designation) will have 12v at all times the ignition is ON. If the wiring is as originally supplied by the factory, C1 will have around 6v when idling - a lighted test probe may not light at this level. None of the other terminals would have power when the engine is running.
For others following this thread and trying to help you should be aware that Jack started another thread concerning the relay a few days ago that may contain pertinent information not presented here: Starter Relay driving me to adult beverages!
The fact that the engine used to start properly for an extended period and now it doesn’t is a key piece of information. Has anything changed? If not, it may simply be a case that your battery is now a little weak and unable to supply sufficient voltage to the Pertronix when under cranking loads. Increased resistance in the starter connection due to corrosion could also have the same effect. However, at this point of the investigation I would focus on getting the requested voltage measurements and trying to determine exactly how your ignition is wired, as it’s quite possible/likely that whoever added the Pertronix made changes from the Jaguar wiring diagram.
-David