Thanks for responding, I really appreciate having someone who knows what they're about coming back to me.

Okay, just working from the beginning, that's where I have my problem...

The horizontal centreline on the diagram certainly looks like it's through the pressure plate holes, but I'm looking at a pressure plate which is many degrees out of sync with that. Which is actually why I'm asking the question.

If both flywheels I have had the pressure plate holes along that line I would go along with that, but even with just one of them not anywhere near it raises a question with me. This is exactly what I mean by having no datum point.