Reading left to right, that's how the trench, which will be a path, has progressed over the last three days. The most satisfying aspect has been how it's filled up with water, which was exactly what was intended. As I was shovelling out the soil and clay, I could see water from the NW bed, (to the left in the photos) seeping out into the trench. Today, I was able to walk on that bed, and whilst it was sodden, (given the rain we've had in Scotland, not surprising), it was not sink-down-12-inches-boggy, as it was a few weeks ago.
I had to dig a subsidiary hole-in-the-ground, NE bed, to take some more water, to the right of the original one, about 4x4ft.
And I'm going to eventually shape both of these holes, perhaps a third one, into a pond, giving it some sloping edges. I quite like that wee island in the middle there, as a bed for bulrushes.
Massive learning curve. The soil underneath the path was clay-soil, that is, not good gardening earth, but not the yellow boulder-clay subsoil either. The boulder clay is impervious, but so was the stuff under the path. That's been part of the drainage problem.
The rest of the drainage problem I think I've worked out, too. To the West of me is an area the size of two plots, and presumably it was two plots once. Then it was made into an area of raised beds, and the ground in-between the beds was paved. The whole area slopes gently down North West. And so, whenever it rains, the water from that whole area, about 600 sq yards, runs down into my allotment's NW bed. The clay-soil under the path running South-North, acted as a dam, and the NW bed became an ungardenable bog.
Next job is to restore the path by filling the trench loosely with bricks, adding a couple of inches of earth (to allow worms and other subterranean beasts free passage from West to East), and topping off with the paving slabs. The whole should be a few inches higher than the old path. The way the NW bed seems to be draining since I cleared out the plug of clay-soil, any further measures there such as a French drain should be unnecessary, but I may need to revisit that.
A great start to 2016, and it's not even 12th Night. You'll notice the blue-ish tinge to the middle and right hand photos in the triptych above. That's because I was finishing at dusk, 4pm-ish. Body's so worked I can barely move tonight. Better than any gym.