It's all right - I think I have cobbled something together to complete
this task. It's just a basic "move the cells around":

Sub MM()
Range(Selection, Selection.End(xlToRight)).Select
num = Selection.Cells.Count
For i = 1 To num - 1
ActiveCell.Offset(1).EntireRow.Insert
Next
For i = 1 To num - 1
ActiveCell.Offset(i, 0) = ActiveCell.Offset(0, i)
ActiveCell.Offset(0, i) = Null
Next
End Sub

I can easily run through it afterwards filling in duplicate values in
the address fields where necessary.

The initial "complete" macro by Dave Peterson is excellent and self-
contained though - and I shall certainly hang onto it for use when a
similar grotty list of postcodes & addresses comes down again for
processing.

Thank you, Rick and Dave, for your time and assistance in this matter.

Steve