Here you can use the following measures in Power Query
#"Added Conditional Column" = Table.AddColumn(#"Changed Type", "Mob", each if [Tag Number] = "v" then null else null),
NewRows = Table.InsertRows(#"Added Conditional Column", 2, {[Mob="15-OHIO", Tag Number="4353654", Electronic ID=1.5, NLIS="", Date="31/05/2015", Live Weight="6", Draft="", Condition store="", Weighing Type="WEAN"]})
in
NewRows
#"Added Conditional Column" = Table.AddColumn(#"Changed Type", "Mob", each if [Tag Number] = "v" then null else null),
NewRows = Table.InsertRows(#"Added Conditional Column", 2, {[Mob="15-OHIO", Tag Number="4353654", Electronic ID=1.5, NLIS="", Date="31/05/2015", Live Weight="6", Draft="", Condition store="", Weighing Type="WEAN"]})
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