Well, if I haven’t found it I’ve certainly made a new one! With Stoneleigh looming every closer I’ve finally gotten around to commencing battle once again. I removed the inlet manifold and had a closer look at what I’d inadvertently photographed earlier.
Zooming in showed it was indeed likely to be the culprit
Ignore the big hole at the top, I made that. It is heavily corroded and there are a couple of pin holes in it. The inside of the head doesn’t look much better
Thanks to Fraser at Blink for confirming the size of the plug! My local Mazda dealership didn’t have a clue