According to The Bookseller, a sixth in the series is to be published, written by Blyton fan, Trevor Bolton:
A sixth book is to be added to Enid Blyton’s Secret Series—the first book in the series to have not been written by the original author.Read the rest of the article, here and refresh your memory about the originals at this Enid Blyton site.
The Secret Series chronicles the adventures of Mike, Peggy, Nora, Jack and their friend Prince Paul. The new story The Secret Valley has been written by life-long fan, Trevor Bolton, who has never been published before.
Award Publications is to publish the new story in paperback on 1st June. The publication date will coincide with the publisher’s launch of the series in a revamped design in paperback. The series was previously only available from Award in hardback. The paperback version has been re-set and digitized and the point size has been made larger.
Bolton was 'discovered' when his manuscript of The Secret Valley was circulated around the Enid Blyton Society—of which Bolton is a member. They thought it warranted publication and forwarded it to Chorion, which holds the rights to Enid Blyton works.