Not surprised in the slightest that the fraudster has an account he's using at Natwest.
Their security when opening new accounts is laughable and their Fraud Team are a bunch of monkeys. You can open an online account with them using only a name and address and they'll give you a £50 overdraft you can use immediatly!
I've just spent 10 months getting two fraudulent accounts opened in my name closed and it took numerous letters and a couple of "the Banking Ombudsman is going to here about this" calls to get the matter sorted.
Just one point which I would imagine you have done already but you can only complain to the Ombudsman after you have sent in a complaint with the bank and they have 'resolved' it and you are still unhappy with the outcome.