The scroll bar is linked to a cell, so my first tries to get it to trigger the macro have been using 

Private Sub Worksheet_Change(ByVal Target As Range)
    Dim KeyCells As Range

    ' The variable KeyCells contains the cells that will
    ' cause an alert when they are changed.
    Set KeyCells = Range("a2")
    
    If Not Application.Intersect(KeyCells, Range(Target.Address)) _
           Is Nothing Then

        ' Display a message when the designated cell has been
        ' changed.
        ' Place your code here.
        MsgBox "Cell " & Target.Address & " has changed."
       
    End If
End Sub
to check for a change in the linked cell. But this doesn't seem to trigger the macro. (The code above works perfectly in a simple worksheet with no ActiveX control, where I simply type a value into the target cell and press enter.)

Can anyone suggest a way to detect a change in the ActiveX control which I can use to trigger the macro?

Thanks for advice