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Richard Borg answered my question (in the same way it was answered here) :
Q. When an enemy leader is alone in a hex do you need to have a leader attached to your attacking unit to score a hit on the enemy leader?
When a leader is alone in a hex and is targeted by Ranged Combat or attacked in Close Combat to hit the leader you need to roll one leader symbol. A leader does not need to be attached to the unit that is attacking the leader to score a hit.