Ken, This might be a bit simplistic, and it is difficult to speculate as you have not given the details.
If your paperwork is right (do you have original copies of documents?), and the DNA is obviously right, it could be that you are incorrectly dismissing the Essex Wilkinsons only because you can't find the right birth or baptism (he may have been registered with forenames the other way around for example, or born out of wedlock and registered with his mmn). It is possible that the Essex and Norfolk Wilkinsons are connected somewhere along the line given the locations.
If you are finding connections to other Wilkinsons who have taken DNA tests is it possible that their paperwork is incorrect?
Does Herbert William's SA marriage certificate give his father's name?
Lastly, I am not sure how you are approaching this search. What do you know to be fact? Are you certain that SA Herbert is YOUR Herbert?
(I originally thought you have original documents passed through the family but on further reading I am not sure).
Apologies in advance if I have misunderstood any of this and am giving dodgy advice.
