Computer Weekly, who first broke this story back in 2009, maintain a comprehensive Horizon timeline. At the time of writing this is their most recent update.
The Justice for Subpostmasters Alliance (JFSA) archive is worth a look, as is the Post Office's archive on the mediation scheme.
Here is my timeline, which borrows heavily from Computer Weekly and adds a few blog posts by me, web articles by various journalists (and the odd barrister), plus links to various transcripts taken from trials and parliamentary debates/evidence sessions.
- Jan 2007: Transcript of judgement in civil case Post Office v Lee Castleton
- May 2009: Bankruptcy, prosecution and disrupted livelihoods – postmasters tell their story [the first piece of journalism on this subject]
- September 2009: Postmasters form action group after accounts shortfall.
- November 2009: Seema Misra's theft case deferred over IT questions.
- November 2010: Transcript of Seema Misra's trial for theft.
- February 2011: Joint BBC South and BBC Surrey investigation. BBC South film here. Transcript of BBC Surrey breakfast show here.
- October 2011: 85 sub-postmasters seek legal support in claims against Post Office computer system.
- June 2012: BBC online: investigators to check Post Office computers
- January 2013: Post Office admits Horizon system needs more investigation.
- January 2013: Post Office announces amnesty for Horizon evidence.
- January 2013: Post Office wants to get to bottom of IT system allegations.
- June 2013: Investigation into Post Office accounting system to drill down on strongest cases.
- July 2013: My take on the Second Sight report into Horizon.
- August 2013: Post Office announces Complaint Review and Mediation Scheme
- October 2013: Former Lord Justice of Appeal Hooper joins Post Office Horizon investigation.
- November 2013: 150 sub-postmasters file claims over “faulty” Horizon accounting system.
- September 2014: Fresh questions raised over Post Office IT system’s role in fraud cases.
- October 2014: Post Office maintains remote access to Horizon branch accounts "impossible" [later retracted in the High Court on 26 Jan 2016]
- December 2014: MPs blast Post Office over IT system investigation and remove backing.
- December 2014: Why MPs lost faith in the Post Office’s IT investigation, but vowed to fight on.
- December 2014: MPs to debate sub-postmaster IT injustice claims.
- December 2014: Transcript of MPs' Westminster Hall debate into the Horizon system.
- December 2014: Two pieces I made for The One Show on the Horizon system.
- January 2015: MPs force inquiry into Post Office sub-postmaster mediation scheme.
- January 2015: A series of BBC Inside Out pieces go out around the country.
- January 2015: Post Office faces grilling by MPs over Horizon accounting system.
- Jan 2015: Private Eye follow up the Westminster Hall debate.
- February 2015: Transcript of the Select Committee hearing into Horizon and the Mediation Scheme [one of the most revealing and under-reported episodes in this whole story - the only time the Post Office CEO has been held to account]
- March 2015: Post Office ends working group for IT system investigation day before potentially damaging report.
- March 2015: MPs seek reassurance over Post Office mediation scheme.
- March 2015: Retiring MP aims to uncover truth of alleged Post Office computer system problems.
- April 2015: Unpublished Second Sight report into Horizon system placed online.
- April 2015: Criminal Courts Review Commission set to review sub-postmasters’ claims of wrongful prosecution.
- April 2015: Powerful Daily Mail article with new case studies
- May 2015: IT system related to sub-postmaster prosecutions under review by CCRC.
- August 2015: Panorama's take on the Horizon story is broadcast, called "Trouble at the Post Office?"
- October 2015: James Arbuthnot takes Post Office IT fight to House of Lords.
- November 2015: The union that represents Post Office sub-postmasters has warned of a problem with the Horizon accounting system.
- November 2015: An email from Post Office IT support reveals a problem with the Horizon system and supporting processes that could lead to accounting errors.
- November 2015: A barrister's criticism of the prosecution of Seema Misra.
- November 2015: Group litigation against Post Office being prepared in Horizon dispute.
- February 2016: Post Office faces group litigation over Horizon IT as sub-postmasters fund class action.
- June 2016: Post Office chairman Tim Parker says there would be “considerable risk” associated with changing its Horizon computer system.
- August 2016: One of the first members of the Justice for Subpostmasters Alliance, Julian Wilson, dies.
- November 2016: The legal team hired by a group of sub-postmasters will take their case to the next stage.
- January 2017: The Post Office admits in court that previous assertions (eg to Panorama) that remote access to Horizon branch accounts was "impossible" was not true.
- April 2017: 1,000 sub-postmasters apply to join group litigation against Post Office.
- April 2017: Investigation into claims of miscarriages of justice in relation to a Post Office accounting system has appointed a forensic accountant firm.
- August 2017: Campaigners submit initial evidence in group litigation against Post Office over controversial Horizon IT system.
- November 2017: The high court judge managing the sub-postmasters versus Post Office legal case over an allegedly faulty computer system tells legal teams to cooperate.
- April 2018: Criminal Cases Review Commission forensic examination of the IT system at the centre of a legal case against the Post Office has raised further questions.
- May 2018: Post Office branches unable to connect to Horizon computer system for several hours after morning opening time.