Rowland, one way to get leeks to get to a decent size is to liquid-feed them.
We used to grow about 200+, and every Saturday would feed them with some nettle tea, mixed with comfrey tea and some 6X diluted in water - all mixed up, and smelling awful, but fabulous for leeks, as the mixture stayed in the folds, and made them very lush! There was a bit of rust on the leaves, but not on the white end.
They all grew from biro-refill size to about 2" diameter, and apart from a bit of hoeing, they just sat there for the whole winter, like leeks do!