From Wikipedia:

Graham Douglas Leonard was an English Roman Catholic priest and former Anglican bishop. His principal ministry was as a bishop of the Church of England but, after his retirement as the Bishop of London, he became a Roman Catholic, becoming the most senior Anglican cleric to do so since the English Reformation. He was conditionally ordained to the priesthood in the Roman Catholic Church and was later appointed a monsignor by Pope John Paul II.

Leonard was a curate in St Ives, Huntingdonshire, and at Stansted, Essex. He then spent three years as vicar of Ardleigh, Essex. In 1957 he became a residentiary canon of St Albans Cathedral and the diocesan director of religious education. His long association with the Diocese of London began in 1962 when, before becoming the Bishop of Willesden (a suffragan bishopric in the diocese) in 1964, he was appointed as Archdeacon of Hampstead and as rector of St Andrew Undershaft with St Mary Axe in the City of London.

Leonard had three episcopal positions in the Church of England, firstly as the suffragan Bishop of Willesden in the Diocese of London and later as the diocesan Bishop of Truro (1973 to 1981) and the Bishop of London (1981 to 1991).

Documents

• A Critique of Eucharistic Agreement
  1975