"(2) Notwithstanding anything in any Act - (a) proceedings for an offence under this Act other than an offence relating to housing benefit or council tax benefit...may be begun at any time within the period of 3 months from the date on which evidence, sufficient in the opinion of the Secretary of State to justify a prosecution for the offence, comes to his knowledge or within a period of 12 months from the commission of the offence, whichever period last expires; and (b) proceedings for an offence under this Act relating to housing benefit or council tax benefit may be begun at any time within the period of 3 months from the date on which evidence, sufficient in the opinion of the appropriate authority to justify a prosecution for the offence, comes to the authority's knowledge or within a period of 12 months from the commission of the offence, whichever period last expires."