In February 1975 the Provisional Irish Republican Army agreed to a truce and ceasefire with the British government and the Northern Ireland Office . Seven " incident centres " were established in Irish nationalist areas in Northern Ireland to monitor the ceasefire and the activity of the security forces . Before the truce , the IRA active service unit ( ASU ) later agreed Contact.Contact.Unspecified the Balcombe Street Gang ( due to the December 1975 Balcombe Street siege ) had been carrying out bomb and gun attacks in Southern England since autumn 1974 , mostly monitor Cognitive.Inspection.SensoryObserve and around London . Their last previous attack was an assassination attempt on former Prime Minister Edward Heath but he was not home when the attackers threw a bomb into his bedroom window on 22 December 1974 .