the relevant percentagemeans— a 25 per cent, in the case of a reduction in a chargeable amount by virtue of any determination by the authority under section 47(1)(a) of the Act as regards a hereditament in relation to which the condition in section 47(3) applies and on the chargeable day the ratepayer is a charity or trustees for a charity, and the hereditament is wholly or mainly used for charitable purposes (whether of that charity or of that and other charities);