On Fri, 15 Jun 2007 02:44:44 +0100, James Farrar wrote:

For many stations outside Z6 the combination of two tickets is actually
cheaper than buying a a single ticket.

The catch is that the combination is only valid on trains stopping at the
boundary station.

The NRCOC says:

19. Using a combination of tickets
You may use two or more tickets for one journey as long as together
they cover the entire journey and one of the following applies:
(a) they are both Zonal Tickets (unless special conditions prohibit
their use);
(b) the train you are in calls at the station where you change from
one ticket to another; or
(c) one of the tickets is a Season Ticket (which for this purpose does
not include Season Tickets or travel passes issued on behalf of a
passenger transport executive or local authority) or a leisure travel
pass, and the other ticket(s) is/are not.

http://www.nationalrail.co.uk/system...misc/NRCOC.pdf

Would a Z1-6 on Oyster not count as a "Season Ticket or travel pass
issued on behalf of a passenger transport executive or local
authority"?

It wouldn't. Appendix A (Definitions) of the same document defines
"season ticket" as including weekly or longer Travelcards.

(OTOH, a Freedom Pass might well count.)