Hello Guy, glad you like WV book and your question is a good one because it touches on the confusing subject of rotation.
You can follow with plants of the same family (last year I grew cabbage then kale and broad then French beans in the same year), so garlic followed by leeks can work though I presume you mean planting leeks (from April sowing) rather than sowing them after garlic. Bandit is a good variety for that.