Thank you for your work - I won't pretend that I can even begin to understand the programming.
but have made one small change which I hope you agree with.
The reason is that I need the data to be listed as 100_1 not 1_100 as per your 2nd code.
Have changed it to
If UCase(Trim(Cells(Rw, "A"))) = "CHAINAGE" Then MyChn = Cells(Rw, "B") & "_"
If IsNumeric(Cells(Rw, "A")) And Cells(Rw, "A") > 0 Then Cells(Rw, "A") = MyChn & Cells(Rw, "A")

This seems to do the job.
Many thanks for your work.
Will hit the feedback icon now.