Tess,
On the family search IGI there is,
birth of Robert James Nichol 3/7/1849, Sunbury Surrey son of Adam & Jane Nichol.
Robert married Annie Williams 1878, South Australia & died 1924.
All information is member submitted so should be treated with caution. For instance, Staines is in Middlesex not Surrey!
I found information on the Taymouth Castle, sailed 7/2/1854 from Plymouth, Devon arrived Adelaide 3/5/1854. Is that the right one?
Looked for their marriage & only found in December quarter 1845 an Adam NICHOLL at Stockport (Cheshire) 19 311. Looked for a corresponding entry for Jane SHIELLS for the same quarter & year but couldn't find her...very strange

Still can't find any of the family members in the 1851 census...again very strange. What about details of your great grandfather, Adam's brother, that might help!
Jane