I have decided to trim them at both ends to make them easier to plant. I haven't done this before but I gather it makes very little difference to the development of the leek and makes them easier to drop into the planting holes. The picture below is before trimming, although many of them had much more roots to trim
And this is it after trimming. I hope I haven't cut too much off, I'm relying on the leek's resilience to carry it through.