(1) If, in a prosecution for an offence against section 622 (Supplying controlled drug to child for selling), it is proved that the defendant—
(a) supplied a trafficable quantity of a controlled drug to a child; or
(b) possessed a trafficable quantity of a controlled drug with the intention of supplying any of it to a child;
it is presumed, unless the contrary is proved, that the defendant had the belief about the sale of the drug by the child required for the offence.
(2) This section does not apply to a single charge under section 629 (Single offence for trafficking etc on different occasions) if the conduct on each occasion to which the charge relates did not involve a trafficable quantity of the controlled drug.