I would suggest they are rotten, leeks should be green/white. Where do you live in the UK and have you had very severe frosts? Our remaining leeks are still fine in the ground.

Leek rust? Sometimes the outer leaves of our leeks get a rust - we strip off some of the outer leaves while leaving them in the ground and it appears to do the trick.

Hello - too high nitrogen and poor drainage? To help you better - when did you plant the leeks? If you are reaping now, then they have been left in the ground too long, or you may have planted too late? Dig them all up and dispose. Try a tub, with base drainage holes, stones/pebbles etc at the base, and john innes no.3 soil. Tub depth should be 18" to get a decent leek, thinned out to 4" apart. Don't feed with miracle grow - they don't need it! Keep watering regular, allow the surface to dry out a little, do not over-water. If we get any heavy persistent rain, then move the tub or cover with a bit of anything.