JK Rowling provides backstory on why the Dursleys dislike Harry Potter so much

On Dudley's birthday, Rowling reveals the story behind Vernon and Petunia's names, and a disastrous first meeting with the Potters

Richard Griffiths as Vernon Dursley, Harry Melling as Dudley and Fiona Shaw as Aunt Petunia 

The Telegraph 23 June 2015

To mark what would be the 35th birthday of Harry's deeply unpleasant cousin, Dudley Dursley, JK Rowling has released some new writing on her Pottermore fan website concerning his parents.

Readers who visit The Cupboard Under The Stairs extract and turn their attention to the table outside it will be rewarded by unlocking the backstory of Vernon and Petunia Dursley. 
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And at The Guardian  JK Rowling reveals why the Dursleys dislike Harry Potter so much