When your broad beans have reached the end, don't dig them up, but cut the plants off at soil level as that leaves the roots with their nitrogen-rich nodules in the ground.
Now you can dibber the holes in the empty bean bed and drop your leeks into them (you may need to pull off some over-bushy root to get them to drop right down in). Then don't back fill with soil, just top up the hole to the brim with water.