On Wed, 5 Dec 2007 05:27:51 -0800 (PST), Matthew Dickinson wrote:

It's not available from the automatic machines, but the ticket seller
is able to issue it as a One Day Travelcard, as long as they set the
origin station to Amersham.

Aha! Thanks. That makes some sort of sense, although it's a bit weird
that they can't just sell it as a standard sort of option. It's also
quite annoying that it's not available at the machines, but at least
I'll know what to suggest to the salesperson next time (if I ever have
to embark on the slow 2.5-to-3-hour journey again - it certainly made me
appreciate the speed of my trip on High Speed 1 the week before!).

Thanks again
Paul