I agree with the leeks and Autumn onions... potatoes take to much to be followed by brassicas in my opinion. Green manures like Winter Tares or Phacalia. :)
A lot of manure goes on top of them there potatoes but i dont believe you get good results from just manure so fertalizer went in and on top of the rows, not all of it will have leeched out.
When i put my cabbages in i put a ring of growmore and a small amount of calcified plus arround each plant up untill now i have had mighty fine results
Below was the last of my spring cabbage the last one being pulled last week