issue with non-English characters passed from .htm page to .aspx page

I have an issue where non-English character (e.g. ñ) being sent from an .htm page to a .aspx page all come through as '?' - and I need to find out how to decode those characters.

my test page & script are:

test.htm  

1    <form action="test.aspx" method="get">
2    <input type="text" name="test_string" />
3    <input type="submit" />
4    </form>


test.aspx (read the querystring test.aspx?test_string=mo%F1ey)
 
1    protected void Page_Load(object sender, EventArgs e)
2    {
3      string myString = Server.HtmlDecode(Request.QueryString["test_string"]);
4      System.Text.Encoding ascii = System.Text.Encoding.ASCII;
5      B ...