The V778 form is the retention certificate issued by the DVLA. All you need to do is give that document to the dealer and they can then register the new car with that number. The previous owner is not involved any more.
For more information about the whole process have a look at the early posts on the Personal Number Plate 'sticky' at the head of this forum.
For more information about the whole process have a look at the early posts on the Personal Number Plate 'sticky' at the head of this forum.