4. Where by reason of the time at which an approved duty begins or ends a member necessarily spends a period of 24 hours or longer away from his usual place of residence, he shall be entitled to an allowance–
(a)not exceeding £60.40 per night spent away from that place of residence, in respect of a duty to be performed in inner London, or of attendance at the annual conference of the Rating and Valuation Association, or
(b)not exceeding £52.95 per night so spent in any other case,
but shall not be so entitled in relation to any such night in respect of which the member’s overnight accommodation is provided at the expense of the Secretary of State.