Where, by any provision of, or of any instrument made under, the Customs and Excise Acts 1979, any spirits become liable to forfeiture by reason of some offence committed by a revenue trader, then—
(a)where that provision specifies the quantity of those spirites but does not specify the spirits so liable, the Commissioners may seize the equivalent of that quantity computed at proof from any spirits in the stock of that trader; and
(b)where that provision specifies the spirits so liable the Commissioners may, if they think fit, seize instead of the spirits so specified an equivalent quantity computed at proof of any other spirits in the stock of that trader.