Hi Angela,
I have seen four entries on Familysearch.org--they stated that he was born in 1857 in the same place you mentioned. He had one more middle name of Francis!
His wife was the same one you stated.
I could not find him in the Birth Indexes either.
I think he may be registered under the different name unknown to us.
I would contact the churches in Kent area--the one that served the area your g grandfather stated that it was his birthplace to see if they have any baptism record on him--hopefully, it will give his birth date.
I have a theory based on one of my relatives who served in the military--he went by four different names at four different times he served in the Army! He had good reasons for doing it. The only trouble is that when it was time for him to collect Army pension--he had to own up to it and told them four names and durations! He did get his pension without any trouble.
It may be the case with your g grandfather--he may had been using this name for some reasons unknown to us--perhaps he wants a better chance of getting good promotions? The result he likes it so much and kept this identity for the rest of his life?
Oh, as to the transcription, it is a quick way to get information if you want to have it in a hurry. For only 4 pounds and one service will transcribe the information you seek and E-mail it to you within 3 days or so.
I think your best bet would be waiting for the marriage entry as it will tell you who was his father then you can try and look for him in the 1850 Census--hopefully, with a wife and children. One of these children will give you an opportunity to obtain a birth record and find out who his parents really were. Then, with a bit of luck, you will find your g grandfather's birth record.
Kind regards,
Tees
