On Thu, 06 Dec 2007 22:37:11 +0000, Paul Corfield wrote:

Interesting to note that these are "fill in" PAYG installations for
Chitern and One and are not an extension of PAYG beyond the stations
where current interavailability applies e.g. Seven Sisters, Tottenham
Hale, Walthamstow Central and South / West Ruislip.

To be fair to Chiltern, this brings PAYG validity to all their
stations in the Travelcard zones, so West Ruislip does seem a natural
limit (for now).