Custom error won't show

 I'm trying to get a custom error page to show error details by using Global.asax to store details about any exceptions, but no matter what I try I can't get the page to display.
I have this in global.asax:
 
void Application_Error(object sender, EventArgs e)
{
Exception ErrorInfo = Server.GetLastError().GetBaseException();
StringBuilder err = new StringBuilder("&lt;strong>Status Code: </strong>").Append(Response.StatusCode)
.Append("&lt;br /><strong>Path:</strong> ").Append(Request.Path)
.Append("&lt;br /><strong>URL:</strong> ").Append(Request.RawUrl)
.Append("&lt;br /><br /><strong>Error Message: ...