Self-assessment for individuals: interest on late payment.

Is the interest on the balancing payment and capital gains tax always calculated on the amount of the reduced payment on account claimed?

In the BPP study text says: ‘Interest is chargeable on late payment of both payments on account and balancing payments. Late payment interest is charged from the due date for payment until the day before the date on which payment is made’.

The ‘day before the date on which payment is made’, refers to:

1) the date of payment of each reduced payment on account?

2) the date of payment of the balance of tax (to calculate the interest on the difference between the original amount of the payments of account and the reduced amount claimed?