The FCA's Client Assets Sourcebook (CASS) provides rules for firms to follow whenever the firm holds or controls client money or safe custody assets. CASS helps ensure the safety of client money and assets if a firm fails and leaves the market.
CASS has many recordkeeping requirements listed throughout. These requirements apply to records regarding allocation of the supervisory function over CASS compliance, elections in relation to certain CASS rules, agreements where firms hold client money as agent, auditor confirmation of the adequacy of firms' systems and controls, the basis of client money calculations, documents evidencing the basis on which money is held, descriptions of asset segregation, terms on which firms hold money and assets in specific situations, details of divestment of assets, evidence of appropriateness of any third parties firms use to hold money or assets and how they review these appointments, client money held for each client, client communications, details of entitlement to client money shortfalls if a bank fails, details of mandates held, details of pools and sub-pools of client money, reconciliations and requirements on specific types of firm under certain targeted rules.
Link to FCA's table of recordkeeping requirements found in CASS: https://www.handbook.fca.org.uk/handbook/CASS/Sch/1.pdf
Link to CASS Handbook: https://www.handbook.fca.org.uk/handbook/CASS.pdf