I don't have the certificate but I see no reason why it shouldn't be this family https://familysearch.org/pal:/MM9.1.1/X3FB-MQ1 Ignore the reference to Middlesex, there is nothing of the kind on the image. 
There is no evidence that your man (or his parents) ever went within thirty miles of Lincoln.
Sarah BLACKBURN (they married 1859) was a sister of an ancestor of mine.

Yes, that seems to make sense to me - which makes 'my' George all the more puzzling!
I did find an entry to a George Lunn having died in Mar 1914, in Doncaster, of the relevant age, which might be a 'fit' and provide the opportunity to work at this from another angle. If I assume that George is definitely one of mine, then I am in contact with one of his descendants, by marriage, so might have to pick their brains a little more!
The part I do still struggle with, is that in all the census information, George still gives his birthplace as being Lincoln?!
Apologies for all the questions and thinking out loud!!
Dave