Hi aquilegia - I don't have white rot (at least as far as I know, on my new plot), so perhaps better waiting for Tim/Emma to come along?
But fwiw, if you don't have white rot (don't introduce it!), keep garlic/shallots/onions strictly in the allium rotation, and be relaxed about the leeks? Mine usually go in after the early potatoes - a big empty space ready just at the time I need a big empty space for the leeks!
I believe garlic is particularly susceptible to white rot, whereas leeks are the opposite, (perhaps because white rot needs a ceratin temperature range ???).
All best - Gavin