Thank you to all. It looks as though it would have been better to give you more information to begin with.
Ann Rees married Samuel Britton in 1834 in Bristol. She had a child (who looks to have been illegitimate) by the name of Martha Giddings Rees in 1832. Martha was born Bristol but baptised on 01/01/1837 in Bitton. The register shows she was 5 years old when she was baptised. Martha married John Pring in 1858 and her marriage certificate shows her father to be Robert Rees. They married in Risca, Newport, Monmouthshire in 1858.
Samuel and Ann had four children and they lived in Bitton near Bristol. Samuel died in 1856 and in 1858 Ann married Harry (Henry) Start. They married in Newport, Mon. There is the strange entry on the 1861 census for them. Harry died in 1867. In the 1871 census Ann is living with her daughter, Martha, in Newport, Mon. In the 1881 census she is living with another daughter, Sarah and her husband William Came in Kensington. She died in 1885 in Kensington.
The only specific place of birth for Ann is Milford, Pembrokeshire. Otherwise, her place of birth is shown as Wales.
The entries on registers etc for Martha may give some clues. I wonder whether she was married before her marriage to Samuel. Was Robert Rees her first husband? Where does the Giddings name fit in? In the baptism entry it suggests she was illegitimate as the father's name has been erased. Just Ann Rees is shown as her parent.
In this family there are many links to Newport, Monmouthshire. As the only entry showing Milford as her birth place is that in the 1861 census showing them both in false names which is suspect. I wonder whether this place of birth might be false too. They appear to be trying to hide their true identity.
It is a brick wall and seems possible the mystery will never be solved.
Jo