Sch 11 para 9
An authorised person may at any reasonable time require a person making such a supply as is referred to in section 23(1) or any person acting on his behalf— (a) to open any machine on which relevant machine games (as defined in section 23A) are capable of being played; and (b) to carry out any other operation which may be necessary to enable the authorised person to ascertain the amount which, in accordance with section 23(3) , is to be taken as the value of supplies made in the circumstances mentioned in subsection (1) of that section in any period.