Thanks guys, it will be hard work, but worth it in the end!
I haven't started off any Leeks yet, I don't eat all that many of them so will probably use some of the trench space for other things, maybe a permenant Asparagus bed?
mc55, I'm sure they would gladly come round and burn your shed to the ground! I have had to promise to pay them with copious amounts of beer to get them to do anything other than just burn stuff though!