I am endebted to Dave Osler on Liberal Conspiracy for the lead. Osler says that Callinicos is Lord Acton's grandson, but a little research shows that he is in fact his great grandson.
Alex Callinicos's mother was the Hon. Ædgyth Bertha Milburg Mary Antonia Frances Lyon-Dalberg-Acton. She was the daughter of Richard Lyon-Dalberg-Acton, 2nd Baron Acton. And he in turn was the son of John Dalberg-Acton, 1st Baron Acton.
Lord Acton, as the first baron is usually known, was often quoted by the radical Young Liberals of the 1960s. He is best remembered today as the originator of the maxim:
All power tends to corrupt, and absolute power corrupts absolutely.