Custom Errors Not Working

so I have the following in my web.config file

  <customErrors mode="RemoteOnly" defaultRedirect="~/Error/Default.aspx">
<error statusCode="404" redirect="~/Error/FileNotFound.aspx" />
<error statusCode="403" redirect="~/Error/FileNotFound.aspx" />
</customErrors>
 when I test this using vs2008 the testing server (local host) does a redirect perfectly if I were to type in an incorrect page.

However I am getting the default IIS error messages for 404 when I put this up on my site.

Any ideas????
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