Thank you Betty, for finding that baptism! I had been searching for hours with no luck, I didn't even think to search Mary Cullen as a first name, and Cheeseman as a surname. Silly me! Looks like Mary was born out of wedlock and took her mothers surname originally and her fathers as a middle name, but then at some point it got swapped over. Very interesting to know!
As for the 1851 census, if that is William and Mary, it's taken me by suprise! I really wouldn't of expected them to turn up in Poplar.
Something that has been playing on my mind, is the Possibility of Mary being the daughter of John Cheeseman and Flora, that Mary Cheeseman was born about 1796 in Hawkhurst, kent. But because of the distance from Canterbury I can't quite tie her in.