Cannot find birth record for Thomas EUSTICE. At time of his marriage, Helston Registry Office, Cornwall,Dec.28 1867, to Matilda GAY, he is shown as an agricultural labourer, living at Wendron St., Helston. His father is shown as John EUSTICE, engine worker in a tin mine.
I am unable to find any verifiable proof, any sort of provable valid record of a Christina Dunstan (or variations) marrying a John Eustice in Cornwall in the 1800s. Nor can I find any such record for a Thomas Eustice living with a John and Christina Eustice (all spelling variations included) in the 1800s.
Thomas Eustice married Matilda Gay in 1867, the Marriage Certificate says, that his father was John Eustice, Wendron St., Helston but there is no Thomas Eustice living with a John and Christina Eustice in the 1851 or 1861 Censuses. Also no birth certificate or christening record exists for such a person. Plus, it seems quite a coincidence that Thomas and Matilda have son, Thomas John born on 3rd. April 1874. Father and son sharing the same birthday?
The Thomas Henry E. mentioned in an answer is not the same person as my checking further has shown.
The options/avenues suggested so far in answers have been explored in over thirty five years of searching.