It seems likely that William Miller is one of the four people of that name baptised (according to Family Search) in Horley as follows:

• 25th January 1750 - parents John & Sarah
• 8th April 1750 - parents Wm & Hannah
• 13th January 1754 - parents Wm & Mary
• 14th March 1759 - parents Philip & Mary

The first 1750 baptism seems the most likely as two of William's children were John & Sarah.