If you've got the root fly overwintering in your soil, apart from giving it a thorough dig to expose the eggs of them (and other nasties like slugs) to the birds, you can use the nemasys grow-your-own thing to try to get rid of them once it warms up and and the eggs hatch. That would be faster and a sure-fire way to get rid of the bu88ers.
Have you had any luck with growing carrots flyaway or resistafly? Or growing them alongside a sacrificial carrot crop (they're supposed to nibble the resitant varieties, think knickers to that for a game of soldiers and gobble up the sacrificial variety of carrots.
Trying carrots for the first time this year, so will be trying this, if it doesn't work next year will be using the nemasys stuff.