Question: In regards to the ITV drama about Colin Howell, what journalist's account was used?

Context articles: The Secret (TV series)
Catherine Steadman
Stephen North
Katherine Kelly (actress)
Tom Hughes (actor)
Colin Howell
Let This Be Our Secret
Kate Buffery
Andrew Buchan
Crime Stories (UK TV series)

The Secret is a 2016 Northern Irish ITV mini-series based on "Let This Be Our Secret", Belfast journalist Deric Henderson's account of the double murder of Lesley Howell and Constable Trevor Buchanan, whose bodies were discovered in a fume-filled car in the garage of a property in Castlerock, County Londonderry, in Northern Ireland in May 1991.
Catherine Steadman is an English actress, best known for playing Mabel Lane Fox in series 5 (2014) of ITV drama "Downton Abbey".
 She also played Nurse Angela Wilson in ITV's 1960s drama "Breathless", Joan Bulmer in the final season of "The Tudors" and Maggie Lewis in the 2016 ITV miniseries "Tutankhamun".
Stephen "Steve" North (born 1965) is an English actor.
 He had his first major role playing Firefighter Colin Parrish in the ITV drama London's Burning between 1990 and 1993.
 He has since appeared in numerous shows on British television, including "Doctor Who Christmas Special 2010", "EastEnders", "The Bill", "The Day Britain Stopped", "Murphy's Law", "Holby City", "Midsomer Murders", "Doctors", "Is Harry on the Boat?
", "" and "Casualty".
 North also played one of the two lead roles in the award winning stage play "Meeting Joe Strummer" along with "Emmerdale" actor Nick Miles.
 He was the solo performer in the original stage adaptation of Nick Hornby's "Fever Pitch", which ran at The Arts Theatre London in 1995.
 He played the role of Ted Narracott in War Horse (play) at the New London Theatre from May 2013 to March 2014 and also in the National Theatre Live broadcast on 27 February 2014 and subsequent Encore screenings.
Katherine Kelly (born 19 November 1979) is an English actress.
 A 2001 graduate of the Royal Academy of Dramatic Art, she played Becky McDonald in the ITV soap opera "Coronation Street" from 2006 to 2012, for which she won Best Actress at the 2009 British Soap Awards and Best Serial Drama Performance at the 2012 National Television Awards.
 Since 2013, she has starred as Mae, Lady Loxley in the ITV drama series "Mr Selfridge".
Tom Hughes (born 18 April 1985) is an English actor.
 His roles include Michael Rogers in "Agatha Christie's Marple", Jonty Millingden in the ITV drama "Trinity", Chaz Jankel in "Sex & Drugs & Rock & Roll", Bruce Pearson in "Cemetery Junction", and Nick Slade in the BBC legal drama "Silk".
 He also played Joe Lambe, the lead role in the 2014 BBC Cold War drama "The Game".
 In 2016, he starred in the role of Prince Albert in the ITV drama "Victoria".
Colin Howell (born 14 March 1959) is a Northern Irish convicted double murderer.
 The murders and surrounding story were the subject of an ITV drama series "The Secret" broadcast in April and May 2016.
Let This Be Our Secret is a true-crime book by award-winning British journalist Deric Henderson about how Colin Howell, aided and abetted by his mistress and fellow-Christian Hazel Stewart, callously killed their spouses and buried the truth for 18 years by making the deaths look like a suicide pact.
Katharine Winifred Buffery (born 23 July 1957) is an English actress.
 She is known for her numerous roles on British television, including the ITV drama series "Wish Me Luck" (1988-1990), BBC miniseries "Close Relations" (1998), Channel 5 legal drama "Wing and a Prayer" (1997-1999) and the ITV police drama "Trial and Retribution" (1997-2002).
 Her stage work includes the 1983 original West End production of "Daisy Pulls it Off", which earned her an Olivier Award nomination.
Andrew Buchan (born 19 February 1979) is an English stage and television actor known for his roles as Mark Latimer in the ITV drama "Broadchurch" (2013–17), as Scott Foster in the BBC political drama "Party Animals" (2007), as John Mercer in ITV drama series "The Fixer" (2008–09), and as William Garrow in BBC period drama "Garrow's Law" (2009–11).
Crime Stories is a British television crime drama series that aired on ITV from 12 November until 7 December 2012.
 Described as "groundbreaking" television, the series was one of a number of series developed by Paul Marquess which makes use of a documentary-style feel and unscripted dialogue.
 The series was panned by critics and viewers alike and was axed by ITV after just one series.
 The series made use of the set previously occupied by fellow ITV drama "The Bill", for which Marquess served as executive producer from 2002–05.