I did in the end implement my own add button which works well. The Prompt property of the AddNewBox style settings is for the text before the butto... Eg.

 <AddNewBox hidden="False" Prompt="Something random">
 </AddNewBox>

 would display the following:

Something random [ BUTTON TEXT ]

thanks heaps though for searching... 

I did get this to work eventually. It is not very clear in the documentation but this is what you do.

<DisplayLayout ...
<AddNewButton Prompt=" " View="Compact"></AddNewButton>
...
</DisplayLayout>
<igtbl:ultrawebband addbuttoncaption="Add" AllowAdd="Yes" AllowUpdate="Yes" CellClickAction="Yes">
<columns
...
</columns>
</igtbl:ultrawebband>

Not sure why they implemented it this way but it works.

Amy

 Here's how you change the prompt and Add New Button caption on the server side.  Put this code in the InitializeLayout method of the webgrid

     protected void myWebGrid_InitializeLayout(object sender, Infragistics.WebUI.UltraWebGrid.LayoutEventArgs e)
    {
        // Configure the Add new row of the uwg
        myWebGrid.DisplayLayout.AddNewBox.Hidden = false;
        myWebGrid.DisplayLayout.AllowAddNewDefault = Infragistics.WebUI.UltraWebGrid.AllowAddNew.Yes;
        myWebGrid.DisplayLayout.CellClickActionDefault = Infragistics.WebUI.UltraWebGrid.CellClickAction.Edit;
        myWebGrid.DisplayLayout.AllowUpdateDefault = Infragistics.WebUI.UltraWebGrid.AllowUpdate.Yes;
        myWebGrid.DisplayLayout.Bands[0].AddButtonCaption = "Add New Button Text goes here";
        myWebGrid.DisplayLayout.AddNewBox.Prompt = "Prompt goes here";

    }