After about 3 hours, and 11 slabs, I tired of the path yesterday, and turned my attention to the French drain in the NW bed. The idea is for it to run diagonally, in the direction of the slope, under the path and into the pond. Most of the drain consists of the pipe you can see in the photo here, with holes cut into it, and will be buried in due course. But the visible bit, that runs into the pond, is an old iron drainpipe, rusted and broken, (you can just see it in the far right middle-ground of the photo).
The bottom of the trench is hard boulder clay. As you can tell, it's already filling up with water. I'm leaving it as-is for a couple of days, to check the level and the slope - I might need to hack a gradient into it before laying the French drainpipe and back-filling it.
So that's how it's looking, on a not-to-scale plan, with the new French drain in royal blue running diagonally to towards the pond, (which is not oval of course, in reality).
The endorphin from 4 hours of hard labour did its work on my head, and I went away utterly shagged out but happy. See, it should be another 4 hours or so to get the path finished and the drain laid, so maybe this weekend, weather permitting. And when that's done I can at last get back to raising the level on the NW bed, get rid of the heaps of earth on the NE bed so that it looks more like an allotment, and less like the Battle of the Somme.