You are not required to have direct access to CHIEF or CDS to be able to make a duty repayment claim (C285), you only need to have specific details available to complete the C285 form and provide supporting evidence.
The C88 can be obtained from the importer’s customs agent who submitted the import declaration on their behalf. If the C88 is not available, an E2 (import entry acceptance advice) may also be used to support a repayment claim. This can be obtained from your customs agent who submitted the import declaration on their behalf. In the absence of C88/E2, a MSS report may be used to support your claim.
You can explore your options in terms of using an intermediary as detailed in the guidance at:
Get someone to deal with customs for you