Paul Corfield wrote:

If you exceed 2 hours between validating on entry and then
validating on exit the system will treat each validation as separate
journeys and will impose the £4 charge for each. I personally
think this is a little extreme as some trips could exceed this journey
time if there is only a slight delay on one leg (Amersham -
Upminster anyone?).

That increases the cost of Circle Line parties. Although quite why
anyone wanted to do them is another matter.

Sid