(1) On the hearing of a charge for an offence under section 49 or 50 of the Principal Act, it shall not be necessary to show that the defendant had not consumed intoxicating liquor after the time when the offence is alleged to have been committed but before the taking or provision of a specimen under section 13, 14 or 15.

(2) Where, on the hearing of a charge for an offence under section 49 or 50 of the Principal Act, evidence is given by or on behalf of the defendant that, after the time when the offence is alleged to have been committed but before the taking or provision of a specimen under section 13, 14 or 15, he had consumed intoxicating liquor, the court shall disregard the evidence unless satisfied by or on behalf of the defendant -

(a) that, but for that consumption, the concentration of alcohol in the defendant's blood (as specified in a certificate under section 19) would not have exceeded the concentration of alcohol for the time being standing specified in subsection (2) of the said section 49 or 50, as may be appropriate, whether generally or in respect of the class of person of which the defendant is a member,