After I moved some of the skinny brick path from the West to the East bed yesterday, I dug a wee hole where the brick path had been. And this is how it looked this afternoon. So my concern was justified: thanks to the phacelia for tipping me off that all was still not well in the Mid and North West beds. And this is very early autumn, mind, it's only going to get worse in the weeks ahead.
So here's the plan:
Along the course of the skinny paths separating the hedge area from the beds, and dividing the Mid and North West beds, I'll dig a ditch about 1ft deep, and fill it with rubble. That will still serve as a path, of course, and should guide the excess water down to the main rubble drain under the central path, and thence into the pond.