William de Haseley

William de Haseley was Sub-Prior of the Benedictine monastery at Westminster and master of the novices. He entered the monastery in about 1266 and compiled Abbot Ware's Customary and acquired some books for the monastic library. He was buried in the church around 1283 when an indulgence was issued by the Bishop of Ely for prayers at his tomb. The location was unknown. 

Further reading

Oxford Dictionary of National Biography 2004

"Customary of the Benedictine monasteries of...Canterbury and St Peter Westminster" 2 vols. edited by E.M. Thompson, 1904