The 1997 book not only had a revealing commentary in the author's hand but also 22 of her own original illustrations.
The hushed saleroom saw two determined bidders vie for the lot and, after the bidding was raised in increments of £25,000, the hammer finally to a telephone bidder.
Although the auctioneers claimed a new auction record for a printed book by Rowling, charity sales are not normally classified as indicative of the market.
In all, the 50 books offered to raise money for English PEN (the writers' charity which campaigns for freedom of expression) raised a total of £439,200.
The second-highest price lot was a copy of Matilda by Roald Dahl with new illustrations by Quentin Blake, which sold for £30,000.
The sale was attended by a number of authors whose works were featured in the sale, including Helen Fielding, Colm Toibin, Jeanette Winterson, Frederick Forsyth, Ben Okri, Margaret Drabble, Andrea Levy, David Lodge, John Banville, Justin Cartwright and Wendy Cope.