We also know that five years after the gift (1163) the monks of Montjoux, who were based at the St Bernard's Pass, built a priory. A prior and twelve monks from Montjoux came to live here. The priory flourished for another two hundred years.

Records in New College, Oxford show that a grant was paid to the church in Havering and the present nave probably dates from this time (1243). This is the first recorded reference to the Church being dedicated to St Andrew.

1391 was a very important year for St Andrew's. The boy King, Richard II (of original 'poll tax' fame), needed finance to pay for the strife between the Crown and the Church. He decided to confiscate lands that were owned abroad; one such piece of land was St Andrew’s.

Because St Andrew's was such a successful establishment under the direction of the monks, William of Wykeham, Bishop of Winchester, bought it from the King to endow his new College in Oxford. Not only did he buy the priory but he also purchased all its lands and manor houses in Havering, but "known as Hornchurch".

Included in the purchase was the advowson; this was the right for New College to appoint the Vicar of their choice. It was therefore at this date (1391) that the Priory became a parish church and is still flourishing as a Parish Church 600 years on.