The marriage certificate is useful for your research and has been worth spending your money. The witness, Jane Pearce TOMKINS.........
Passengers on the "Cartvale" include Jane P TOMPKINS, 20 years, native place Herefordshire.
Census 1871 has Jane Pearce TOMKINS, 17 years, granddaughter of Jane and James, at Richards Castle, Herefordshire.
That supports the idea that Annie REID, arr. Oct 1874, "Cartvale" is your Annie.
If Herefordshire was closer to Kent you might research the possibility that Annie and Jane are related, but at this stage I would not follow that.
The marriage record, 1875, that you have looks to be a transcription from a church record to the civil register. Everything is written in the one hand....no signatures.
The church marriage register for Primitive Methodist Church, Sydney St, would have the original. The original church record would have the same information as this transcription. The signature of William COOK might be useful for comparison if you ever locate him. But not worth spending money on. I am assuming that you live in NZ, and can access the church records.
The Primitive Methodist Church, Sydney Street, can be found in Paperspast so was a separate church to the one in Webb Street, and existed at the same time.
The burials of George, Annie and Lena all involve the Primitive Methodist Church (or ministers). This family has an ongoing association with the church.
Church records, births, burials, marriages, would be useful for you to browse to locate GARDNER and COOK names.
Death certificate Annie, 1896 has sons 19, 15 years...daughters 14, 12, 8 years
Death certificate George GARDNER, 1905 has son 24 years, daughters 23, 21, 16 years.
So, between 1896 and 1905.........a son born ~ 1877 has died......or
The son born ~1877 is son of Annie......... but not of George?
If William COOK was around for any time you would think that Annie has a child with him. William, a bachelor, would probably give his name to a first son. Children born to Annie seem to have two given names.
Look for William COOK, maybe with a second given name, born ~ 1877 Wellington..........baptism recorded in Sydney Street or Webb Street Primitive Methodist Church.
I am looking for the fate of Jane Pearce TOMKINS/ TOMKINS etc. in New Zealand, not seeing anything so far.
New Zealand Area Directory 1886 at Webb St
COOK Mrs, laundress
No house numbers here, but these names are sequential, suggesting they are adjacent houses.