(During the Civil War two thirds of the Cowdray estate was sequestered from the 3rd Viscount Montagu, and the house was garrisoned by Parliamentary forces.
The position remained vacant for some time, during which the church was governed by the Old Chapter, a body of archdeacons and deans created by Smith's predecessor, William Bishop, as a standing council for his own assistance, with power, during the vacancy of the see, to exercise episcopal ordinary jurisdiction.
[3] Leyburn was one of the divines recommended to the authorities at Rome in 1657 as successor to Richard Smith, Titular Bishop of Chalcedon, as Vicar Apostolic of England.
In a particular congregation for English affairs held in the Quirinal Palace on 6 August 1685, the Propaganda, on the relation of the Cardinal, elected Leyburn vicar-apostolic of all England, and the pope gave his approbation the same day.
[6] Leyburn tried to moderate James II's zeal for the Catholic cause, and he told the king that the fellows and students of Magdalen College, Oxford had been wronged, and that restitution ought to be made to them on religious as well as political grounds.
[6] When the Glorious Revolution broke out, Leyburn and Bonaventure Giffard were seized at Faversham on their way to Dover, and were under arrest when the king was brought there.
On 9 July 1690, he and Giffard were liberated on bail by the court of queen's bench, on condition that they transported themselves beyond sea before the last day of the following month.
[6] Leyburn translated into Latin Sir Kenelm Digby's treatise on the soul, under the title of ‘Demonstratio Immortalitatis Animæ Rationalis,’ Paris, 1651 and 1655.