A previously unseen sketch for the third Harry Potter book will go on sale today in London for 8,500 pounds sterling. Art experts estimate that the lucky buyer could re-sell the coveted piece for a five-figure sum.

The sketch's illustrator, Cliff Wright, who also drew the artwork for Books 2 and 3, commented: "In some ways I prefer the sketch to what ended up on the cover. It's more dynamic: in the final version, the hippogriff is not moving quite so fast. JK Rowling herself faxed over an image of what she thought the hippogriff should look like."