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I'm a bit late to the party, but I've got access to Belford Erskine Street baptisms where there are 200 or so entries between 1793 and 1820, none of which are Johnson, or similar.

Sorry it's a negative answer.

These records are on the NDFHS website. I can fully recommend joining, it's £15 a year, and their online records are very useful if you have ancestors in Northumberland or Durham.

As you say the y-dna won't help.
There is no evidence that my GGM had any other children after my GM, she was 38 at that time. The earlier children were all from her first husband. A few years later she married her second husband and was living with him and his child (not hers) with the earlier children in orphanages or living with siblings. All of this was investigated at depth several times years ago and again in the light of dna. The first husband is not the father (or grandfather which was an examined if slight possibility), and neither is the second husband. None of the suspected dna matches (the Unknown group) have close enough matches to be able to assume any surnames. The Unknown group, where they have trees, have ancestors in the area outlined above in the late 19th century and beyond, so almost certainly a local rather than a seaman.
So what I am left with is a man between the ages of 20 and 60, with an unknown name, maybe one of the Potentials' names, but maybe not as they may be a couple of generations away, who was in or within traveling distance of Sunderland in mid 1892.
Ideally they had another child whose descendant gets a dna test, they would then be my H2C with about a 120 cM match. There is currently no-one anywhere near that, who is an Unknown, on Ancestry, MyHeritage, Gedmatch or FamilyTreeDNA.
That leaves me with a lot of Potentials who are probably at least one or two generations removed from my GGF so I need to reduce the number down to less than ten, say, and examine them closely.
I am currently looking at BanyanDNA as a way of testing the likelihood of connections but it will be a lot of work as it is really does the inverse of what I need. It tests given Hypotheses and what I need is it to tell me the most likely Hypothesis.
Hence the original question, I'm looking for another angle of attack.
 

Does anybody know of any Books, Academic Papers, Websites or anything else that has a methodology for discovering the identity of unknown ancestors (specifically in my case Great-Grandfather)
One of my two main goals when I first did the dna test was to discover the identity of my Grandmothers father which is blank on her birth cert. I was hoping for a bunch of 2nd cousins to pop up and point directly, but no such luck.
Over the years I have collected about 100 matches who I think are related to me through this unknown GGF and built several trees connecting some of them together but I currently feel that although I've got a lot of data I'm still stumbling around trying to get lucky somewhere.
What I need now is a methodology I can apply to this data and testing techniques to somehow validate what is currently just an intuitive feel.
For example if I take two dna matches and find a common ancestor for them that may be on the same line to me or it may not, but what are the odds of each and how are those odds altered if I find three or four or more going to the same common ancestor. Intuitively I feel as though three or four matches connecting to the same ancestor would be on the path to me but I would like some maths to back this up.

Anybody ?

I am looking at a couple of entries in FreeBMD thus -

{SURNAME} Jean              {Mother} Sunderland 1a 1201
{SURNAME} Elizabeth J     {Mother} Sunderland 1a 1201a

I thought originally this was twins but now I'm wondering if the J stands for Jean and the suffixed entry is a name change.
Anybody come across this before and know the answer ?

PS
The names are redacted as they could well be still alive. It's a year not included in the GRO website so I can't look up the full name.