15. Where—
(a)immediately before 4th April 1977 a person is absent from Great Britain other than temporarily; and
(b)as respects a period before and including 3rd April 1977 he satisfies the conditions then in force for, and is entitled to receive, payment of an amount by way of a benefit or allowance or an increase of a benefit or an allowance under the Act in respect of a child who is ordinarily resident in Great Britain; and
(c)would cease, as from 4th April 1977, to be entitled to that payment by reason of the fact that he does not satisfy one of the conditions for receiving such a payment, namely, that he is entitled to child benefit in respect of that child,
that person shall, for any period beginning not earlier than 4th April 1977 during which he would, or could had he made an appropriate claim, be entitled to child benefit in respect of that child were he not absent from Great Britain, be treated as so entitled while he continues to satisfy all other conditions applicable to such a payment (including making contributions to the cost of providing for that child over and above those that would have been required for the purpose of satisfying subsection (1)(b) of section 3 of the Child Benefit Act) unless subsequent to 4th April 1977 he becomes ordinarily resident in Great Britain.