The liability of underwriters is limited to the reasonable charges payable by the insured to their auditors for producing and certifying any particulars or details contained in the insured's books of account or the business books or documents or such other proofs, information or evidence as may be required by the underwriters in connection with a claim under this section of the policy but in no case shall this exceed ....... in any one period of insurance, the sum insured stated in the schedule.