Well the plot thickens....... I decided to order the death certificate in 1853 and guess what.....its the right one for my Mary Robinson. She died 20th Jan 1853 at 5 King Street, Deptford aged 40 the wife of William Henry Robinson a costermonger from Typhus for 2 weeks. Everything matches what I know and the informant was Mary Elizabeth Robinson who I presume is their daughter who I have only seen recorded as Mary Ann before and just Mary afterwards.
So the question now is how do William & Mary go on to register the birth and baptism of another child in 1854, more than a year after she died!?!

Do you think it's one big cover up by their daughter who is now calling herself Mary Elizabeth Robinson?
Mary Ann Robinson, the daughter, went onto marry a William Podbury in 1861 and in the 1871 census has a son recorded as living with her called Robert George Podbury born c.1856 and I'm now wondering if he's actually the Robert George Robinson born in 1854 supposedly to William & Mary Robinson.....