Toby thank you. This has been one of my dead ends but this seems to be my Elizabeth. She appeared on all censuses till 1891. In 1851 she was aged 47 and in 1871 she was 67 so I am happy. If you are able to check if there is any other info about the Sheldons in Great Tew, e.g. siblings' baptisms or a marriage between John and Elizabeth, the parents, I would be so grateful. Will have to try to get to Oxford and look it all up but not likely to happen soon. Many thanks for your help, Carl.