Murray has asked me to look at some of these details with fresh eyes and all of that! Not sure it will help as you guys have already gone through plenty here.
Can I ask some questions first? On the shipping manifest in 1849 and the arrival in NZ of a John Johnston from Wigton. Does it actually mention Scotland or is it just simply Wigton? I couldn't see a possible John J on the 1841 census in Wigtownshire, but lots elsewhere in Scotland for that year with undeterminded birth place (so frustrating the 1841 census

).
However, there is a possible Johnston family in Wigton, Cumberland to consider in 1841 (if there is no reference to Scotland on that 1849 list).
William Johnston 50 inn keeper
Elizabeth Johnston 45
John Johnston 15
Charles Johnston 13
Bridget Johnston 10
Joseph Johnston 5
www.familysearch.org/ark:/61903/1:1:M73F-DF5This John is the only one showing in 1841 as not born in the county. If he lived there from very young, not surprising he would refer to himself as born in Wigton. The rest of the family were all born in the county.
Parents with youngest Joseph in 1851
www.familysearch.org/ark:/61903/1:1:SG24-JF8Murray, the other question I asked you via PM is whether you know the name of Elizabeth Bell's parents back in Scotland. You have mentioned that you have more info for her.
I see from the list of children for John and Elizabeth (Bell) that the names of Elizabeth and William could fit well here:
• Elizabeth Johnston, born 1859 (folio #334) Dunedin
• Martha Johnston, born 1861 (folio #439) Taieri
• Anne Johnston, born 1862 (folio #325) Christchurch*
• Jane Johnston, born 1864 (folio #611) Dunedin
• William Johnston, born 1866 (folio #979) Warepa
• Catherine Johnston, born 1868 (folio #1291) Kaihiku, Warepa
• Mary Wylie Johnston, born 1870 (folio #1084) Oamaru*
• Joan Johnston, born 1871, Jan. to March quarter (folio #1443) Popotunoa
*These are not likely part of the family because of their place of birth.
Monica
