The Greater London Authority (Referendum Arrangements) Order 1998

6.  For the purposes of paragraph (1) of rule 31 (voting procedure), the same copy of the register of electors may be used for the poll at the referendum and the poll at the ordinary election and one mark may be placed in that register under paragraph (1)(d), or in the list of proxies under paragraph (1)(e), of that rule to denote that a ballot paper has been received in respect of both polls; except that, where a ballot paper has been issued in respect of one poll only, a different mark shall be placed in the register or in the list of proxies so as to identify the poll in respect of which the ballot paper was issued.