You only send the export slip if YOU are exporting the car yourself and are still the keeper which you are not.
I Phoned the DVLA before I sold mine and they said the buyer will need the whole document for his import.
In the end I made my buyer fill in the v5 as per normal sale and took a copy, i then sent the copy and a copy of the sale receipt signed and dated by both to the DVLA with a covering letter, the new keeper then had the original V5 complete.
all seemed to work as I have received my letter saying im no longer the keeper of the car!
hope this helps