MI5 The UK's domestic security service acts in complete secrecy as the UK's secret police force.
One can safely assume that MI5 officers use a similar network of informers as the main stream domestic police force but without the same minimal level of scrutiny afforded by the police complaints commission.
Indeed one former MI5 officer talks of whistle blowing on any corruption being met by persecution, imprisonment or even mysterious deaths.
Looking closer at the case of Andy McCardle; the testimony of this former MI5 agent explains how he was framed and his murder covered up.
One witness states that MI5 officers threatened to kill his family with a fake gas explosion if he did not agree their demands and confess to a crime he did not commit.
Link to daily mail article of MI5 murder threat.
These two articles tell of a sorry state of affairs with a domestic secret police force free to carry out crimes including murder and framing innocent victims without any scrutiny.