How can I see all of my clients' cash holdings in one place, to ensure all clients are invested where there should be?

You can download a client report from the Platform: launch Platform Services and then select each client's account's "Business Reporting". Once there, run the "Clients By Adviser Summary Of Assets" business report. This report will show each client accounts' total value and how much of that value is in product cash, expressed as both £ and %. Note that this report does not currently include Corporate or Trustee Investment Portfolio accounts. Running the "Assets Under Administration" business report will give Corporate/Trustee Investment Portfolio case details, including amount held in cash.