The Minister shall determine the hourly rate applicable to SRM inspectors, and may determine different rates for different classes of SRM inspector, having regard to the level of qualifications and experience, and to the salary costs (including any local variations in salary levels), of inspectors in that class, and may vary any rate so determined where, having regard to the factors set out in paragraphs 3 and 4 below, it appears to him to be necessary and equitable to do so.