From what I can see, Charles' parents had their first child in 1758:
John, son of William & Sarah Chalk, baptised 5th October 1758 in Bower Chalke.They then baptised children at fairly regular intervals until 1772, when their last child, Anne, was baptised.
William Chalk, of Bower Chalke, married Sarah Joliffe, of Fifield Bavant, on 4th October 1756 in Fifield Bavant. The banns were recorded at Bower Chalke.
This marriage fits perfectly with what we know about your William & Sarah.
William Chalk married Sarah Downes on 27th December 1769 in Downton, Wiltshire. Both were of the parish of Downton.
I don't think this marriage fits.
Queenie