The site for talent agency Alphabet Kidz reports that child actor Jamie Oram has been cast as Harry in the fourth episode. Oram has previously appeared in a number of commercials, but the part of Harry will be his first role in television drama.
Actor Daniel Mays will also appear in the episode as Harry's father. Mays is known for his role as Jim Keats on Ashes To Ashes and will appear in forthcoming BBC sci-fi drama Outcasts.
Harry's mother will be played by Emma Cunniffe, who has previously featured in episodes of The Bill and Casualty and appeared alongside former Who star Christopher Eccleston in the 2002 show Flesh and Blood.
A casting call on Spotlight describes the character of Harry as "a troubled young boy who has a nervous disposition".
Production on the episode - apparently titled 'What Are Little Boys Made Of?' - is currently ongoing in Cardiff.