No trace of a John Martin born on August 29, 1818 but I think below may be the two births you may have found
Norval Douglas MARTIN was born on February 24, 1824, in Hackney, Middlesex
Father a painter abode Hackney Rd. However when he married his father was a Boot Maker, who also acted as a witness
Ann MARTIN was born on September 12, 1826, in London, London
Father a painter abode Hackney Rd.
Based on Norval’s middle name maybe these are the parents?
James Martin married Elizabeth Douglas 7 Jan 1819 St Bartholomew the Great, city of London.
One of the witnesses James Douglas.
1841 - St Giles Without Cripplegate
James Martin 55 Scotland Painter
Elizabeth Martin 54 Scotland
Ann Martin 14 London
1851 – 14 Whiteiwas St. Cripplegate
James Martin 69 Born Ayrshire Pensioner 75th Foot
Elizabeth Martin 57 Born Renfrewshire
Still in the area in 1861 James a Painter of houses, ages 78 & 75.
However whether the above is correct or no not?
I would mention however there was another couple in the same of the same name, that James was a carpenter and they had a son John in 1821?
The other thought is that maybe John was born before he married Elizabeth?
Hope some of this may help?
Colin