Where a person has paid an amount to the Commissioners by way of tax which was not tax due to them, they shall be liable to repay the amount to him.

The Commissioners shall only be liable to repay an amount under this paragraph on a claim being made for the purpose.

It shall be a defence, in relation to a claim under this paragraph, that repayment of an amount would unjustly enrich the claimant.

No amount may be claimed under this paragraph after the expiry of six years from the date on which it was paid, except where sub-paragraph (5) below applies.

Where an amount has been paid to the Commissioners by reason of a mistake, a claim for the repayment of the amount under this paragraph may be made at any time before the expiry of six years from the date on which the claimant discovered the mistake or could with reasonable diligence have discovered it.

A claim under this paragraph shall be made in such form and manner and shall be supported by such documentary evidence as may be prescribed by regulations.

Except as provided by this paragraph, the Commissioners shall not be liable to repay an amount paid to them by way of tax by virtue of the fact that it was not tax due to them.