(1) In this section 'specified period', in relation to a year of assessment, means the period beginning with the specified return date for the year of assessment (within the meaning of section 959A) and ending 3 years after the time when a return under Chapter 3 of Part 41A for the chargeable period is delivered to the Collector-General.
(2) For the purposes of this section -
(a) where the relevant time (within the meaning of section 579B) falls within the period of 12 months beginning with the 11th day of February, 1999, the relevant period is the period beginning with that day and ending with the relevant time, and
(b) in any other case, the relevant period is the period of 12 months ending with the relevant time.
(3) This section shall apply at any time on or after the 11th day of February, 1999, where -
(a) section 579B applies as regards the trustees (in this section referred to as 'migrating trustees') of a settlement, and
(b) any tax, which is payable by the migrating trustees i
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