I'm sorry it's taken me so long to respond to your posting!
Boy in Darkness was a novella which Peake wrote at the time he was working on the Titus books. It depicts Titus in a surreal and sinister adventure, completely independently of the Gormenghast books.
As far as I know there has never been either a radio or television adaptation of the story, but I could be wrong!
The last of the trilogy is Titus Alone, a book which takes place outside the confines of Gormenghast, and which is more realistic and downbeat than the first two books. It suffers from the fact that Peake was ill when he wrote it, but at the same time it deals with issues which are deeper and more immediately relevant than those of the first books.
Thanks for visiting the site.