However, it's not all bad news, since I did plant my spare Leeks in a big (35-litre) pot and they have done a lot better.
Eight Leeks crammed into one pot were never going to be huge. Furthermore, the soil in the pot had already been used to grow a crop of potatoes, and had not been refreshed. I now need the pot for this year's potatoes, so I have pulled up the Leeks.
Actually, if I had been planning to grow these as "Baby Leeks", I would be well pleased. Other than their diminutive girth, there is little to complain about.
Here they are, cleaned up ready for using in the kitchen:
The moral of this tale is definitely "Less is More". The concept of under-cropping (growing one crop underneath another) sometimes works, but you have to choose the right crops! I was greedy, hoping to get two crops at once, but it didn't work this time.