Hi,

What happens when you open my sheet with macros enabled and why do you think it doesn't work.

You should first see the formula sum in G1 resulting in £22.84. The formula being =SUM(E4:E13)

When you click the button the formula is changed to =SUM(E14:E1048576)

Do you not see this?

The macro in question is at what's called the Module Level and is

Sub FillUp()
    Dim x As Long
    x = Range("E" & Rows.Count).End(xlUp).Row + 1
    Range("G1") = "=Sum(E" & x & ":E" & Rows.Count & ")"

End Sub
If you're adding it to another workbook then just copy that macro to a new Module in your workbook. ALT-F11 to go to the VBE, find your workbook name in the Project pane on the left, select the 'ThisWorkbook' item and then from the menu pick Insert Module

Then just double click the Module1 object and in the code area on the right paste the code above. Finally on the worksheet add a button, any shape will do, right click the shape, choose 'Assign Macro' and select the 'FillUp' macro.