Thanks all.
My gggf (Matthew Thomas/Thomas John's father) was also called Thomas. His occupations were recorded as Labourer at 17 and Gardener at 27.
My ggf (Matthew Thomas/Thomas John) is the firstborn, with 3 siblings, whose births are all correctly recorded in GRO with their names they were known by and mother's maiden name.
Now I see his 1883 birthdate is estimated from census records (8yo in 1891 and 18yo in 1901), age at marriage (24) and age at death (41) so the GRO 1882 birthdate works fine!

He left England and emigrated with my ggm to South Africa. He was a carpenter. He died before 1939.
Family are adamant his name was Matthew, uncles and cousins have been named after him
But I'm positive this 'Thomas John' is the birth certificate for him. I've ordered it. It's so unlikely it could belong to someone else. But I am trying to use this evidence to support a claim to British citizenship (for my mother), so I need it to be irrefutable.
I will start looking for baptism/christening records
Registration district: Penzance
Birthplace: Ludgvan, Cornwall
Ecclesiastical Parish: ST JOHN
Thanks for all your replies.