Retrieves one of the authenticated client's payees with all details. Deleted payees are returned with isDeleted true; a payee that does not exist or belongs to another client returns 404. channel is how the payee was created: Portal, Api, or Brokered when an operator created it on the client's behalf. accountNameVerification is the Confirmation of Payee result for GB sort-code payees; it is null when no verification applies or the record cannot be found. Returns 503 when the verification service is unavailable.