One of the sources cited by Dr. Matchett is the "Walker Report", a document prepared in 1980-1981 by a then senior member of the British Security Service (MI5) and following a lengthy legal case a redacted copy is now in the public domain. It is relatively short and opens with:
In January 1980 the Chief Constable commissioned a report - known as the Walker Report - on the interchange of intelligence between Special Branch and CID and on the staffing and organisation of units in C1(1) in Crime Branch.
Link:http://www.patfinucanecentre.org/policing/walker-report

The legal case is explained here:https://caj.org.uk/2018/05/01/psni-a...-rights-group/

Two journalists from 'The Guardian' have written an overview, it starts with:
A secret MI5 report that resulted in Northern Ireland’s police covertly prioritising intelligence-gathering over fighting crime has been made public after almost 40 years. The report resulted in detectives of the Royal Ulster Constabulary (RUC) – now the Police Service of Northern Ireland (PSNI) – being ordered never to arrest a suspected terrorist without consulting the force’s intelligence-gathering section.

Detectives were also told that anyone who was arrested could be recruited as an agent rather than charged with a criminal offence.
As a consequence, a number of British agents are now known to have been involved in murders, bombings and shootings, while continuing to pass on information about their terrorist associates.
Link:https://www.theguardian.com/uk-news/...thern-ireland?

There are a number of similar MI5 reports cited which remain secret.

Informant handling is always an activity fraught with risks, even more so in CT / COIN and 'The Troubles' lasted a very long time, with the UK fighting a very capable enemy, PIRA and often violent Loyalists who waged their own campaign.