Hi Anne, it is never a good idea to grow vegetables in the same are two years running normally you would operate a 4 year crop rotation cycle:

Year 1 potatoes
Year 2 legumes
Year 3 brassicas
Year 4 roots, which your leeks are viewed as.

There are several reasons for this particular rotation:

You dig a lot of compost or well rotted manure into the potato trenches
The legumes return nitrogen to the soil in their root nodules
You then lime the soil for growing your brassicas and add more compost
Your root veg., in general does not want a rich soil doing better on a lighter sandier soil.
If you grow the same crop on the same piece of land two years running you run the risk of there being a build up of pests and diseases in the soil.

Thank you for this very thorough advice. I am growing vegetables in my very small garden on an even smaller plot so am going to have to think this one through!

Why not plant the leeks where the broccoli was an vice versa?

Thanks for your suggestion. Am now going back to the drawing board and looking at how to split the small area up into four sections ready for rotation! No doubt will be back eventually with questions about how to grow potatoes!