CH231100 | How to do a compliance check: information powers: rules that apply to all notices: accountants’ working papers: introduction
From HM Revenue & Customs · Compliance Handbook
Where facts and circumstances allow, you can seek access to an accountant’s working papers.
The following documents are protected and may not be included in an information notice if they are the property of the auditor or tax adviser concerned
auditor’s papers
relevant communications giving or obtaining tax/duty advice.
But if they contain essential linking information, you are allowed limited access to that information.