Hi Mike, a few things first - how old are they? Because if they're quite mature and planted in the ground then you might get some new shoots near the base soon. If they are fairly young then I would've thought they're probably dead, sorry! Also, have you been watering them regularly during the drought we've had? It could just be that you're another unlucky one who's cordylines suffered the harsh winter. All I can suggest is waiting to see if you get any shoots growing lower down the trunk, and plenty of water during dry hot spells.

If they were damaged by the winter then I would've expected new shoots by now, as most 'dead' cordylines are starting to recover now.