On November 27, 1975, Guinness Book of Records editor Ross McWhirter was assassinated by two members of the Provisional IRA outside his North London home.

McWhirter, who together with his twin brother Norris had gained fame through founding the Guinness annual and appearing on the BBC TV show Record Breakers, was an outspoken critic of Republican terrorism, advocating a tougher response by the British government.

He had recently offered a reward of £50,000 for any information that might lead to the arrest of an IRA cell carrying out a bombing campaign in London.

Police investigate Ross McWhirter's murder at his house in Enfield

Four members of the cell were captured two weeks later, after a six-day siege in Balcombe Street, Marylebone. Two men, Harry Duggan and Hugh Doherty, were charged with the assassination of McWhirter and with nine other murders.

[Read more: April 24, 1993 - IRA’s Bishopsgate bomb devastates the heart of the City of London]

Duggan and Doherty received life sentences with ‘whole life’ tariffs, but were released in 1999 under the terms of the Good Friday Agreement.