(a) In this section references to tax being payable shall, except where the context requires otherwise, include references to tax being payable by a person to withdraw from that person so much of a tax advantage as is a refund of, or a payment of, an amount of tax, or an increase in an amount of tax, refundable, or otherwise payable, to the person.
(b) For the purposes of this section the date on which the opinion of the Revenue Commissioners that a transaction is a tax avoidance transaction becomes final and conclusive is -
(i) where no appeal is made under section 811(7) against any matter or matters specified or described in the notice of that opinion, 31 days after the date of the notice of that opinion, or
(ii) the date on which all appeals made under section 811(7) against any such matter or matters have been finally determined and none of the appeals has been so determined by an order directing that the opinion of the Revenue Commissioners to the effect that the transaction
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