Hi Lesley,
Something wrong on FreeBMD for that marriage - there's 1 groom too many for the page number 768 he's listed on.
I wonder if he should be on page 769, where one groom is missing
(Mary Jane Jesty, John King, Mary Ann Parker)
Having said that, when I look at the original image for William, I definitely think it's 768 or 758.
Can you post the 1871 reference, please, or Mary's age and birth place?