Thanks for the clarification - my comments were based on the ninth impression so they say it is 18.
On this topic, I did enquire with the UKBA regarding how to study for questions on topics like this one where the materials have changed, or the situation has changed after publication. The official answer I was given is that questions on these topics should not come up in people's tests.
However, I would advise caution myself and recommend learning the materials as reproduced from the latest version of the study materials for the test.
I would be interested to hear from any users who have had this question come up in their test recently though, to confirm if this question has been taken out of circulation or not.
George