In March 1550-1 he became a licensed preacher, and about the same time was chaplain to the Duke of Suffolk, and tutor to his daughter, Lady Jane Grey.
[2][3] In August 1552 Haddon was given a prebend in Westminster Abbey, and in October became dean of Exeter, though the patent was not signed until 8 January in the following year.
He preached before the court in Lent 1553, when, as John Knox relates, "he most learnedly opened the causes of the bypast plagues, affirming that worse were to follow unless repentance should shortly be found".
[2] On the accession of Mary I he was one of the six champions in the convocation of October 1553 who maintained the cause of the Reformation in five days' disputation on the real presence.
Among manuscripts preserved at Corpus Christi College, Cambridge, is a letter De Matrimonio addressed to him, probably by Martin Bucer.