I noticed that the tunnel was wet a couple of days ago, and thought a hose had come adrift - but after looking at them, they where all tight... and the dripping was coming from the bottom of the airbox.
What I can't understand is that it didn't leak when I got it - I flushed it out and it seemed fine. And the liquid that leaked out seemed to be a greenish colour, rather than the orange of the coolant. Weird.
It looks as if the lower part of the core has failed, and a whole load of crusty residue had built up round the fins - again, this wasn't there when I stripped it down the first time to repaint the air box.
I can only assume it's built up over the time it's been in the car, and that I somehow damaged the core putting it back in. And that the car is obviously Vulcan, as it has green blood.
Ah well - It's all cleaned up now, and I've got a new core ordered and on the way.