721. Payments on account of income tax.

The following provisions apply to any person ('the taxpayer') as regards a year of assessment1 if, as regards the immediately preceding year:


    (1)     he is assessed to income tax2 in any amount;


    (2)     that amount ('the assessed amount') exceeds the amount of any income tax which has been deducted at source3;


    (3)     the amount of the excess ('the relevant amount') is not less than such amount as may be prescribed4;


    (4)     the proportion which the relevant amount bears to the assessed amount is not less than such proportion as may be so prescribed