669. Assessment where loss of tax discovered.

If an officer of Revenue and Customs discovers, or the Commissioners for HM Revenue and Customs themselves discover1, as regards any person ('the taxpayer') and a year of assessment2 that:

  1.  

    (1)     any income which ought to have been assessed to income tax, or chargeable gains which ought to have been assessed to capital gains tax have not been assessed;

  2.  

    (2)     an assessment to tax is or has become insufficient; or

  3.  

    (3)     any relief which has been given is or has become excessive,

that officer or, as the case may be, the Commissioners for HM Revenue and Customs, may make an