C# usercontrol LinkButton CommandArgument blank after grid subheading

Question

I have an asp:Gridview that displays the amount of time a staff member spends on a project and their comments. I wish to make the date a sub heading within the grid, so it is not displayed on every line of the grid. The grid cannot be sorted. The grid is within a usercontol, which is within another usercontrol. The columns from the database query are:

1. Date
2. Project ID
3. Project Name/code
4. Staff comment
5. Time spent

The gridview is defined as:

  <asp:GridView ID="grdTimeSheet" 
      runat="server" 
      AutoGenerateColumns="False" 
      CssClass="mGrid" 
      AllowPaging="false" 
      AllowSorting="false" 
      BorderStyle="None" 
      GridLines="None" 
      DataKeyNames="ItemID" 
      CaptionAlign="Top" 
      CellPadding="1" 
      CellSpacing="1" 
      ShowFooter="True" 
      OnRowDataBound="grdTimeSheet_RowDataBound" 
      Width="100%" 
      OnRowCommand="grdTimeSheet_OnRowCommand" >

Within the GridView I have a Link Button:

<asp:LinkButton 
      ID="ItemLinkButton" 
      runat="server" 
      CommandArgument='<%# Eval("ID") %>' 
      CommandName="Select"  Text='<%# Eval("ProjectNo") %>'  >
 </asp:LinkButton>

The OnRowDataBound event in the code behind checks to see whether the date on the record has changed and if so, adds a new row to the grid to display the date. This process displays okay.

The OnRowCommand checks the CommandName value and if it equals Select I retrieve the project ID from the CommandArgument. This works fine for all rows except the one directly under the date sub heading.

I have used the following code to create the sub heading:

            Table tbl = e.Row.Parent as Table;
            if (tbl != null)
            {
                GridViewRow row = 
                   new GridViewRow(
                      0, 
                      0, 
                      DataControlRowType.DataRow, 
                      DataControlRowState.Normal);

                TableCell cell = new TableCell();
                cell.ColumnSpan = this.grdTimeSheet.Columns.Count;
                cell.Width = Unit.Percentage(100);
                cell.Style.Add("font-weight", "bold");
                cell.Style.Add("color", "black");
                HtmlGenericControl span = new HtmlGenericControl("span");
                span.InnerHtml = currentDate;
                cell.Controls.Add(span);
                row.Cells.Add(cell);
                tbl.Rows.AddAt(tbl.Rows.Count - 1, row);
            }

The problem is the CommandArgument is returning blank for the first record under the subheading. All other links are okay. If I get rid of the subheadings everything works okay.

 protected void grdTimeSheet_OnRowCommand(
           object sender, GridViewCommandEventArgs e)
 {
     if (e.CommandName == "Select")
     {
         selectedItemEvent(int.Parse(e.CommandArgument.ToString()));
     }
 }

I have found a couple of examples of people with the same issue, but no real answers.