I tried planting leeks in three potato bags this year, in decent compost and regularly watered.
The spacing was just a few inches - thirteen per bag, and the yield isn't that bad, although the plants nearest the wall they were up against, are much smaller.
The yield isn't bad for the area they're in, with the plants maxing at around an inch diameter, so I'll try again next year, in a different position!
Leeks are a fabulous veg, and a staple here! I saw some in a local shop at 90p each yesterday...!