Please can you tell me, which one is correct? Fred Smith, the builder, who or Fred Smith the builder, who
Your first example. In parenthetical expressions such as this you cannot omit either the first or second comma, unless the interruption to sentence flow is minimal, which I would argue it is not here. See Strunk & White's Elements of Style freely available on Wikisource (Chapter II.2.3 - parenthetic expressions)
What is correct depends on the rest of the sentence: In this case, the builder is not parenthetical. It is necessary to indicate which Fred Smith is which. The phrase who had built the Town Hall is parenthetical in this case. In this case the entire phrase the builder who built the Town Hall is a single parenthetical phrase. But you can also write it as: This time, you are making the builder and who built the Town Hall, separate parenthetical phrase. That independence is clearer if there is no obvious relationship between the two parenthetical phrases:
My opinion is that you need no comma after "builder," because "who" refers to the builder. So, "Fred Smith, the builder who did whatever he did ... " blah, blah, blah.
