Keeping Benefit Data on one system will not stop Identity Fraud. At the moment Tax Credits are paid by Customs and Revenue, Other benefits, such as Disability, Incapacity Benefits are paid by the Department for Work and Pensions.
I work as a Fraud Specialist for DWP and I am always amazed that some of these double benefits are not picked up quicker than they are.
If you have no joy after the interview, my advice is to write to your MP and insist he/she investigates. Believe me, all hell breaks loose when an MP letter arrives.