It is 2. Here's the reasoning :
- under clause 51.2, interest is paid on late certified payment.
- 'interest' is not Defined Cost and hence not part of the Price for Work Done to Date
- nor is late payment a compensation event, so it cannot adjust the (target) Prices.
Hence, while it is part of an assessment for the 'amount due', it does fall into the above categories. Once paid, you can forget about it from a contract admin point of view.