This is the view from google earth. This image is before my time, I'm guessing it was taken a couple of years ago. That's my plot edged in green. To the left is the raised beds area. You can see by its proportions that it's two plots joined together. A large part of it is paved, (that near circular area in the middle, and the paths between the beds, and that area in its bottom right corner. The red arrow shows the direction of the slope. It's a gentle slope, probably a couple of feet from the bottom left to the top right in the photo. But the thing is, almost all of the rain that falls on that area, about 5400sq ft, just trickles slowly over the paving (and the impacted grass areas), and into my plot.
I don't know when the two plots were paved over, and presumably nobody thought about the effect it would have on poor old Plot 79. The Predecessor must have been flummoxed. When the plots to his left were being cultivated, or even if they were standing full of weeds, they would have soaked up all the rain that fell on them. And then someone had the bright idea of communal raised beds, (that IS a bright idea, actually) with a load of paving in between the beds (a bloody stupid idea), and the plot's been waterlogged ever since. P had made valiant attempts to deal with it, there was a dyke and a wee ditch along the Western edge. He must have been trying to deal with it for several years. Along the dyke, completely hidden by vegetation, was an entire carpet runner, from someone's hall or staircase. He must've put it there to keep the water out and maybe keep the weeds down, but the water kept coming and the weeds grew right through it.
Time and fortune have passed the situation on to me. I'm not going to be bloody beaten.