Is artistic director of the acclaimed early music ensemble The Dufay Collective with whom he has toured the world and produced a series of critically acclaimed recordings. He is also a regular performer with leading early music groups and has recently formed a new band The City Musick to explore early English music.

Time is otherwise spent as a specialist session musician, music consultant, composer and researcher. William is director of the  Medieval & Renaissance Studies Post Graduate Course at the Guildhall School of Music & Drama and at the Royal College of Music. William also composes and arranges music for film and TV, including the Pride and Prejudice and Elizabeth: The Golden Age. He has a long association with the Shakespeare's Globe Theatre, having composed scores for over fifteen productions there as well as being a regular Musical Director and Historical Music Adviser.