I am creating a set of drop down lists on a page with the following code:

Table table = new Table();
        TableRow trow = new TableRow();
        TableCell tcell = new TableCell();
        DropDownList ddl;
        Label lbl;
        table.ID = "tblItems";
        table.BorderColor = System.Drawing.Color.Blue;
        table.BorderStyle = BorderStyle.Solid;
        table.BorderWidth = 1;

        int i = 0;
        foreach (DataRow row in dtColumns.Rows)
        {
            trow = new TableRow();
            tcell = new TableCell();
            tcell.ID = "Cell" + i.ToString();
            tcell.Width = 120;
            ddl = new DropDownList();
            lbl = new Label();
            lbl.Text = row["Column_Name"].ToString();
            tcell.Controls.Add(lbl);
            trow.Cells.Add(tcell);
            tcell = new TableCell();
            ddl.DataSource = dt;
            ddl.DataTextField = "Choice";
            ddl.DataValueField = "Reference";
            ddl.DataBind();
            ddl.SelectedValue = row["Column_Name"].ToString();
            ddl.ID = "ddlFields" + i.ToString();
            tcell.Controls.Add(ddl);
            trow.Cells.Add(tcell);
            table.Rows.Add(trow);
            i++;
        }
        hidColumns.Value = (--i).ToString();
        plhColumnMatch.Controls.Add(table);
        plhWorksheets.Visible = false;
        plhColumns.Visible = true;

On the page is a button and it OnClick event contains the following code:

for (int i = 0; i <= intColumns; i++)
        {
            DropDownList ddl1 = new DropDownList();
            string strValue;
            Button btn = (Button)Page.FindControl("Button1");
            ddl1 = (DropDownList)plhColumnMatch.FindControl("ddlField s0");
            strValue = ddl1.SelectedValue;

The last piece of code generates the error. I have tired referencing a control (a button) that was added at design time but I get the same. I have tried variations, such as:
strValue = (plhColumnMatch.FindControl("ddlFields0")as DropDownList).SelectedValue;
but I still get the same.

I am using VS2005 with sp1 and it using localhost/http not file

Thanks
Robert