After some reminders from @Boots to try a stepdown transformer first and some links for cheap ones from ebay, Low and behold everything is now working correctly. 

You’re a legend, kind Sir! This problem was starting to drive me mental.

It’s a 300W version and seems to hold up well, even during multi-ball, so can’t go wrong for $35 delivered!
I will put it in the base of the machine, once I tidy up/re-cable tie the bottom panel loom that I ripped apart in desperation.



All of my weird readings have now completely vanished. Uncontrolled solenoids have exactly 38VDC, GI doesn't flicker when solenoids fire and have a perfect 12VDC to my A2 PSU.

Lesson learnt. If it has US transformers, install a stepdown.

Now I can spend some enjoyment time, playing my Volcano, the way it is supposed to play.