(b) by calling in a financial guarantee if the contractor has submitted one to the contracting  authority; (c) by taking legal action. II.24.1  The contracting authority and the European Anti- Fraud  Office may check or require an audit  on the  Implementation of the FWC . This may be carried out either by OLAF’s own staff or by  any other outside body authorised to do so on its behalf. II.24.2  The contractor must keep all original documents stored on any appropriate medium, including  digitised originals if authorised under national law, for a period of five years starting from the  payment of the balance of the last specific contract issued under this FWC. II.24.3 The contractor must grant the contracting authority’s staff and outside personnel authorised  by the contracting authority the appropriate right of access to sites and premises where the  FWC is implemented and to all the information, including information in electronic format,  needed to conduct such checks and audits. The contractor must ensure that the information  is readily available at the moment of the check or audit and, if so requested, that information  is handed over in an appropriate format. II.24.4  On the basis of the findings made during the audit, a provisional report is drawn up. The  contracting authority or its authorised representative must send it to the contractor, who has  30 days following the date of receipt to submit observations. The contractor must receive the  final report within 60 days following the expiry of the deadline to submit observations.  38  The audit procedure is initiated on the date of receipt of the relevant letter sent by the  contracting authority. Audits are carried out on a confidential basis.  Such checks and audits may be initiated at any moment during the provision of the services  and up to five years starting from the payment of the balance of the last specific contract  issued under this FWC. If the leader does not pay by the due date the whole amount, and if the amount due cannot be offset  or can only be offset partially in accordance with Article II.23.2 (a), then the contracting authority  may claim the amount still due to any other member or members of the group by respectively  notifying them with a debit note in conformity with the provisions laid down in Article II.23.2. If the contract is signed by a group (joint tender), the group is jointly and severally liable under the  conditions set out in Article II.6 (liability). The contracting authority shall send the debit note first to  the leader of the group. Any partial payment is first entered against charges and interest on late payment and then against  the principal amount.  If the contractor does not honour the obligation to pay the amount due by the date set by the  contracting authority in the debit note, the amount due bears interest at the rate indicated in  Article II.21.8. Interest on late payments will cover the period starting on the day after the due date  for payment and ending on the date when the contracting authority receives the full amount owed. II.23.3 Interest on late payment  II.23.4$_{ }$Recovery rules in the case of joint tender  II.24 CHECKS AND AUDITS 