Automatically update a cell with last modified date and time

I am maintaining a Test Environment Catalogue, with various users editing it. It is change controlled in SharePoint, however, I'd like to insert a cell labelled "Last Updated" - and the cell should reflect the last date and time the excel sheet was edited. (Only edited... not viewed... )

Columns B:K have content that could be edited
I have added a row with cell A1: Last Updated
And B1: <Cell that will contain dd/mm/yy hh:mm>

I have tried to create a module:

Public Function ModDate()

ModDate = Format(FileDateTime(ThisWorkbook.FullName), "dd/mm/yy hh:n ampm")

End Function

But this does not work

And I tried:
Private Sub Worksheet_Change(ByVal Target As Range)

Dim WorkRng As Range
Dim Rng As Range
Dim xOffsetColumn As Integer
Set WorkRng = Intersect(Application.ActiveSheet.Range("C:k"), Target)
xOffsetColumn = 1
If Not WorkRng Is Nothing Then
Application.EnableEvents = False
For Each Rng In WorkRng
If Not VBA.IsEmpty(Rng.Value) Then
Rng.Offset(0, xOffsetColumn).Value = Now
Rng.Offset(0, xOffsetColumn).NumberFormat = "dd-mm-yyyy, hh:mm:ss"
Else
Rng.Offset(0, xOffsetColumn).ClearContents
End If
Next
Application.EnableEvents = True
End If
End Sub


But could not quite understand how to modify this code for my sheet....

Any help from your end would be greatly appreciated....

Many Thanks,
Naz