Many thanks for your reply.
This question has just come up in JAR exams in Switzerland.
A friend of mine has just missed the last JAR subject needed (by only one point), because of this question. The exams are necessary to convert an IACO licence to a JAR licence, and secure a job on offer.
Said friend is an experienced jet Captain and tried to convince the Examiner that the answer was 6NM, not 12NM, but could not explain it to the satisfaction of the Examiner. Language differences may have contributed to this.
Over the last weekend the question has been discussed at length with various friends. Some can understand that the answer is 6NM but cannot understand the theory behind it (I didn't understand it fully myself), hence my original post.
Hopefully with your information an appeal for the JAR subject will be successful.