Information Powers: TMA70/S20: Definitions: Particulars
Section 20(1) enables you to require the taxpayer to furnish ‘particulars’ as well as documents.
This matches the power of the Appeal Commissioners to issue a notice under Regulation 10 for documents or particulars.
The ‘particulars’ that can be sought could be the answer to specific factual questions but could also extend to the production of documents that do not already exist. Thus you could call for balance sheets if not already provided with accounts and could require the production of a statement of assets and liabilities or full capital statements by the taxpayer.
Note, however, that the Section 20(1) cannot be used to require taxpayers to bind themselves to a certified statement of assets etc.