From the Council of London or Westminster, A.D. 1200

Canon XI. Let a man not contract with any blood-relative formerly of his wife; similarly neither let a wife contract with any blood-relative formerly of her husband. And let one received in baptism not contract with the daughter, born before or after, of the one baptizing or receiving. Nor let any marriage be contracted without a triple public denunciation in the Church, nor if the persons are unknown. But neither let any persons be coupled in marriage except publicly in the face of the Church and in the presence of a priest; and if it shall have been done otherwise, let them not be admitted anywhere in the Church except by special authority of the Bishop.

You see that the presence alone of the priest is required; nothing is said of his special action; but clandestine marriages indeed are prohibited, yet they are not invalidated.