In message of Tue, 19 May
2009 00:11:29 in uk.transport.london, asdf
writes
On Mon, 18 May 2009 11:00:51 +0100, Walter Briscoe wrote:

OUT IN Station
Y - KXSP NR
- Y KXSP LU
N - Farringdon
N - Barbican
Y Y Moorgate
N N Liverpool Street

As it happens, my investigations were backed up by a Z1-3 Travelcard and
the ticket stopped working at Moorgate after it had been rejected at
Liverpool Street; it worked at Moorgate after rejection at Farringdon.

It probably stopped working at Moorgate after it had been accepted at
Moorgate.

It was accepted twice outgoing at Moorgate!
Paul, Thanks for the advice. On Saturday, I will try both London
Terminals between Moorgate and Paddington with a Travelcard backup.
The other half - lacking a Travelcard - will use Oyster PAYG. I will
report experience, some time after next Tuesday.
--
Walter Briscoe