Like all Alliums, leeks produce seed in a round "fireworks display" of small pods on the top of a hollow stem. If the "pod" in question is a big teardrop on a stem, that is actually a flower bud, and will need to open, bloom, and produce ripened seed. I would store ripened seed in a labelled zipper bag in the fridge, along with a paper envelope full of powdered milk or silica gel. Plant them when Steragram says, then.

Powdered milk! Does this keep them dry?

Yep! Silica gel is more efficient that way, but powdered milk is usually easier to get.

There must be loads of absorbent things that would be less prone to mould than dried milk though? Dry wallpaper paste? Rice?

Rice might work, though not as dry as freeze dried milk--and probably just as prone to mold. Mold is a good argument for searching out silica gel. It should be available at a hobby or craft store.