The waters have receded considerably but it is still too claggy to work, so the new raised beds will have to wait a while longer.
I measured the space where I am planning to put the raised beds and it is a bit smaller than I thought, so I have two options:
1] saw a foot off each long plank and put up with less growing room, or
2] move the rain pond a little further down the plot and lay the new beds lengthways. Just need to decide which one's easiest and best.
I've just had a brilliant idea! I could move the rhubarb, which currently runs lengthways along the fence, to a transverse row running from path to fence. That would give me more room for the raised beds and it would also be growing something that won't be affected by the onion white rot. And the other soil that had some white rot will be safely (hopefully) tucked away under the raised beds. I'm planning to lay the wooden frames on the existing surface with a lining across the bottom, with new soil on top, rather than dig them in, as I need the height to lift the beds out of the floodwaters.
Just planning/thinking aloud, it will probably be something completely different by the time it gets done, but it looks like it could be a neat solution to the problem.