11. Where an earner has been employed in an employment during a period in which employment of that description was contracted-out but such employment was not (apart from this regulation) contracted-out in relation to him, and the Secretary of State is satified—
(a)that it was not contracted-out in relation to him solely because he was not during that period a member of the relevent scheme; and
(b)that he was not during that period a member of that scheme solely because of inadvertence,
he may determine that that earner was in contracted-out employment by reference to that scheme during that period.