C# – Trying to convert Global.asax 1.0 file to 3.5 Issues with Application_Error + Session and Redirect

So in the Global.asax is this:

protected void Application_Error(object sender, System.EventArgs
{
Session["CustomError"] = Server.GetLastError();
Server.ClearError();
Response.Redirect("~/ErrorPage.aspx");
}

And in ErrorPage.aspx is this:

private void Page_Load(object sender, System.EventArgs e)
{
Exception currentException = ((Exception)Session["CustomError"]).InnerException;

// Writes the error message
if (currentException != null)
txtErrorMessage.Text = currentException.Message;

// Loops through the inner exceptions.
currentException = (Exception)Session["CustomError"];
while (currentException != null)
{
message.Append(currentException.Message).Append("\r\n").Append(currentException.StackTrace);
message.Append("\n==============================================\n");
currentException = currentException.InnerException;
}

As this is old 1.0 code it barfs when converted to a 3.5 Global.asax file. It tells me that "Session" is not available and also that I can't redirect?? I think one of the issues may be that there is also an error being thrown from Application_Start. But if I comment out all the application start code I still get errors but they never get redirected to the error page.