I know this post is old, but I've just found it. I have the Dennis family from Gosberton in my tree, which John Dennis and Mary are you looking for? I have a John c1871, Son of John c1833 who married Mary Nelsey in 1858. It may be that John junior's Father had died before he married, so his Mother's name was on the certificate instead.
Let me know and I'll see if I can help further.