So the mission to get the engine started began in earnest. After studying the wiring diagram I had a few ideas of where and what to check, which meant stripping the loom apart (as shown in the pic).
After 5.5hrs I managed to locate a broken wire giving an intermittent voltage to the coils, not a constant 12V. The wire had broken internally with no indication of damage to the insulation. I temporarily bypassed the break and we have a spark! Yay!

Now that it all actually works, the job for the next session is to tidy up the wiring, rewrap the loom and get the dash back to how it was.