Carol Klein in the Guardian Magazine's Gardening section two weeks ago (the butler is a bit behind on his reading):
Carrots are prone to attack from carrot fly, but the flies never reach more than a foot or so above the ground, so you can beat them by gravity in elevated pots or raised beds.
Bloody big flies!
I bought Carol's book Grow Your Own Veg on Friday. Not that I ever will, you understand—but it's a very pretty book.