Hi, there is a baptism record in the SFHG Sussex Baptismal Index for a William Myles, baptised 22 Jan 1764 at Holy Trinity, Hurstpierpoint to parents Matthew & Sarah Myles.
There are also baptisms with parents Matthew & Sarah Miles at Hurstpierpoint as follows:
Matthew, bp. 4 Oct 1767
John, bp. 22 Nov 1769
Sarah, bp. 16 Oct 1778
Jane, bp. 4 Sep 1780
There doesn't appear to a baptism recorded around this time for a William Miles/Myles with father John. As Sillgen mentioned, a John & Mary Miles have children baptised at Woodmancote around this same time, but no William.