Dibbered leeks.

... leave the hole open and it will allow the leek to expand as it grows.

Simples.

 
Ahem, is all expansion already finished for leeks? Mine got to non-invisible seedling size this year before planting out, but most are only about as big around as a finger. I think I might have managed a few thumbs! Mine are Lyon 2, which seem to suggest they could be harvested Sept-Dec.

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They will still be growing but slowing down now. As they are fairly small, they may continue to grow for longer before going to seed. You could try feeding them if you have anything suitable, a general fertilizer or nitrogen rich one would be good. Robert Milne in his book 'Organic Vegetable Growing' suggests using urine diluted 1:1 at 1 litre per sq metre.