The latter, I suspect.
http://www.theguardian.com/notesandq...-20949,00.html
But then I would have if I'd been answering it ... I was aware that this information was out there but the only bit of it that I remember offhand is that the modern GCHQ has, more or less, the historic role played by MI8. Presumably the reason GCHQ doesnt have the colloquial title of MI8 in the same way that the Security Service (MI5) and the Secret Intelligence Service (MI6) do, is that GCHQ was a distinct branch of the service in its own right, and had MI8's duties merged into it, rather than actually being a continuation of MI8.