Chittlehampton baptisms are online at https://www.genuki.org.uk/big/eng/DEV/Chittlehampton/ChittlehamptonBaptisms and a Walter Acland MAIR(E) and wife Honor baptised 2 children there in 1776 and 1777, but not your William. Walter & Honor (SLADER) married in 1771, so I would think that possibly they had other children between 1771 and 1776. Perhaps check nearby parishes?