The only grounds of appeal against an identity unknown notice is that to comply would be unduly onerous.
You must consider the effect that complying with a notice will have on the person receiving it. The most common effect is time; however, there could be other issues such as business disruption or the cost of retrieving documents for example.
You should consider these issues and weigh them against the benefits that the information or documents will bring to the check.
If the burden is disproportionately greater than the benefit to be gained, it would be considered unduly onerous and any appeal would be upheld by the Tribunal.