Kia ora Ian, great to hear from you.You certainly have the right name and location as John Windsor Luxton of Northlew, Devon, married Grace Davey of Sheepwash, Devon, way back on 28 May 1835, in Sheepwash, and they landed in Christchurch New Zealand in 1850, starting the family line here.And 150+ years later they have a lot to answer for! (just joking)
It sounds like Jack and Margaret Luxton were your parents' visitors, and Margaret's son later helped Margaret put all her research together into the book recently launched here in NZ.
I will answer you personally with a bit more detail later.
Regards Ann Luxton great-great-granddaughter of John Windsor Auckland, NZ