Sadly, the author who did win of the Guardian award - Henrietta Branford - passed away the next year.
Transcript follows. Image supplied by James Roach.

Image: James Roach
Transcript
Edinburgh
11th April 1998
Dear Thomas,
Thank you very much indeed for your card wishing me luck in the Guardian Children's Fiction competition. Unfortunately, I didn't win — a writer called Henrietta Branford did. But she's very nice, so I smiled bravely and said "Congratulations" and tried to look as though I didn't mind at all.
I am very flattered indeed to hear that "Harry Potter" is the best book you have ever read. There will be seven Harry Potter books altogether, and the second one ("Harry Potter and the Chamber of Secrets") will be in the shops at the start of July.,
Thank you again for writing. (Your handwriting is excellent, by the way).
Yours sincerely,
J. K. Rowling (Jo)