Hi,
I found these using Ancestry, in the "London, England, Marriages and Banns, 1754-1921" they have the image of the wedding cert.
Oops I forgot to mention the witnesses, in case they help,
Gilbert Lewis SOUTHGATE
Eliza? COLLINS
John GREEN gives LEWKNOR as his residence so it looks like the right couple. I think that there are many reasons that they might have married away from home..... Mary could have left the area after being left some money and John missed her and went after her.....Maybe the marriage was frowned upon so they eloped.....etc
Why couldn't they have just written down their life stories for us to read, especially when they sound extra interesting and have puzzles that we may never know the answers to

Daisy