Switch where cell contains json data

Paste into the advanced editor and that's it.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WilDSUfJOrQSSoQUl+XlAujpGySBGycrYxEInRskQzLIEsoxALFOTWqVYnWilSKA6p8TizGQg7ZJfnoeqE6YqCkmVb2YKmiJTmPGmZkDlsQA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [Customer = _t, #"Account Type" = _t, Town = _t, #"Peak Buiness Months" = _t]),
    #"Parsed JSON" = Table.TransformColumns(Source,{{"Peak Buiness Months", Json.Document}}),
    #"Expanded Peak Buiness Months" = Table.ExpandRecordColumn(#"Parsed JSON", "Peak Buiness Months", {"0", "1", "2"}, {"0", "1", "2"}),
    #"Unpivoted Other Columns" = Table.UnpivotOtherColumns(#"Expanded Peak Buiness Months", {"Customer", "Account Type", "Town"}, "Attribute", "Value"),
    #"Removed Columns" = Table.RemoveColumns(#"Unpivoted Other Columns",{"Attribute"}),
    #"Changed Type" = Table.TransformColumnTypes(#"Removed Columns",{{"Value", Int64.Type}}),
    #"Added Custom" = Table.AddColumn(#"Changed Type", "MonthNumber", each [Value] - 348 + 1),
    #"Removed Columns1" = Table.RemoveColumns(#"Added Custom",{"Value"}),
    #"Added Custom1" = Table.AddColumn(#"Removed Columns1", "Peak Month", each Date.ToText( #date(2020, [MonthNumber], 1), "MMM" )),
    #"Removed Columns2" = Table.RemoveColumns(#"Added Custom1",{"MonthNumber"}),
    #"Grouped Rows" = Table.Group(#"Removed Columns2", {"Customer", "Account Type", "Town"}, {{"Peak Months", each (_)[Peak Month], type table [Peak Month=text]}}),
    #"Added Custom2" = Table.AddColumn(#"Grouped Rows", "Peak Month", each Text.Combine( [Peak Months], " " )),
    #"Removed Columns3" = Table.RemoveColumns(#"Added Custom2",{"Peak Months"}),
    #"Renamed Columns" = Table.RenameColumns(#"Removed Columns3",{{"Peak Month", "Peak Months"}})
in
    #"Renamed Columns"

Best

D

Hi antonyf 

I've adjusted the file to accommodate an extra table