After previous unsuccessful attempts to fix my coolant leak I'd only managed to make it slightly worse. Luckily, whilst pondering my options after the latest failed fix I stumbled across a photo I took which appeared to show that the core plug in the head was leaking. I decided it must be that as it did fit where the leak was and to be fair I was running out of other causes. With Stoneleigh looming ever closer I removed the inlet manifold (completely this time) and got looking.

Hmm... definitely a candidate, maybe I shouldn't have painted them and it would be easier to see. Zooming in shows this:

It looks like the right hand side has perished. As I'd already taken the punt and bought some core plugs anyway, I set about removing it. This is what I had:

Ignore the big hole in the top - I made that whilst removing it. It does look heavily corroded and there are a few pin holes in it. If it wasn't the source of the leak, and I can't see how, it needed replacing anyway. I should probably plan to remove the engine and replace all of the others next winter.

Here's the state inside the head:

Urgh. I've cleaned out the rubbish as best I can and am now leaving it to dry before fitting the new core plug with sealant.

Thanks to Fraser at Blink for confirming the size of the plug! My local Mazda dealership didn't have a clue.