The following is an excerpt from page 4 of the Warhammer Annual 2002 release with an expanded explanation by Gav Thorpe regarding Characters repositioning after a successful charge.

On the turn you charge with a unit containing characters you can't reposition them within the unit. If they end up in base-to-base contact with the enemy they can fight and take part in challenges; if not they'll have to wait.

In your following turn(s) you can move them within the unit to get into base contact with the enemy as explained on page 97 of the Warhammer rulebook.