ASP.NET custom error page - Server.GetLastError() is null

I have a custom error page set up for my application:

<customErrors mode="On" defaultRedirect="~/errors/GeneralError.aspx"
/>

In Global.asax, Application_Error(), the following code works to get the exception details:

  Exception ex = Server.GetLastError();
  if (ex != null)
    {
        if (ex.GetBaseException() != null)
            ex = ex.GetBaseException();
    }

By the time I get to my error page (~/errors/GeneralError.aspx.cs), Server.GetLastError() is null

Is there any way I can get the exception details on the Error Page, rather than in Global.asax.cs ?

ASP.NET 3.5 on Vista/IIS7

Looking more closely at my web.config set up, one of the comments in this post is very helpful

in asp.net 3.5 sp1 there is a new parameter redirectMode

So we can amend customErrors to add this parameter:

<customErrors mode="RemoteOnly" defaultRedirect="~/errors/GeneralError.aspx" redirectMode="ResponseRewrite" />

the ResponseRewrite mode allows us to load the «Error Page» without redirecting the browser, so the URL stays the same, and importantly for me, exception information is not lost.

OK, I found this post: http://msdn.microsoft.com/en-us/library/aa479319.aspx

with this very illustrative diagram:

_{(source: microsoft.com)}

in essence, to get at those exception details i need to store them myself in Global.asax, for later retrieval on my custom error page.

it seems the best way is to do the bulk of the work in Global.asax, with the custom error pages handling helpful content rather than logic.