The amount chargeable is called the cash equivalent of the benefit of the loan. This is the difference between:
The use of official rates prevents arguments over what would be
a commercial rate of interest on an equivalent loan.
Official rates are prescribed by the Treasury by means of
Statutory Instrument.
There are tables of official rates at
EIM26104 to EIM26106.
Detailed instructions on how to calculate the cash equivalent
are at
EIM26200 onwards.
As regards the treatment of belated interest payments, see
EIM26255 to EIM26257.