On Fri, 16 Oct 2009 08:12:41 GMT, Neil Williams wrote:

Availability: In the country-bound direction travel is permitted on
any train on any day. In the London-bound direction, travel is only
permitted on trains timed to depart after 1200 (midday) on Mondays to
Fridays No restriction applies on Saturdays, Sundays and Bank
Holidays."

Well... if I'd read this before my other post. Seems that they
already do do such a season ticket.

But only for certain journeys - there's a Flexitime season for
Brimsdown to Hertford East, as mentioned, but not for Brimsdown to
Ware, or Tottenham Hale to Hertford East, or Hertford East to R3456.
This seems very odd.