On Sun, 11 Jun 2006 15:46:03 +0100, Clive Page wrote:

This is presumably intended to cover the case of passengers from
the outer zones (say Bedford) buying (a) a cheap day return
Bedford-Elstree, and also (b) a London one-day travel-card from Elstree.
My understanding is that these could legally be combined on a train
which did not stop at Elstree, since one is a zonal ticket and the other
is not,

Nope - the rule you are thinking of says that *both* tickets must be
zonal.