You only pay one fee for the whole exchange.
You haven’t mentioned all the other sums of money before which you kept parting with.
With regard to the money that you have sent in dribs and drabs, there is nothing you can do about that other than report it to the police and action fraud. It will have gone to a bank account unless that account has now been closed. If you transferred it by Western Union, it will have gone.
The bank can’t actually legally do what they have done.
The legal situation is that the £15,000 in your account was mixed with your own money and hence, they have not just returned the stolen money, they have, as you quite rightly say, taken your own money.
Further, they can’t just take whatever money is there and leave your money behind because legally, (an odd concept to get your head round) of all the money in your account, they don’t know which is your money and which is stolen money. It’s not a mathematical thing, it’s to do with the same physical money being taken out or not.
You are entitled to be reimbursed whatever money was in your account at the time the stolen £15,000 came in. I think you are saying that was £1600 because it isn’t clear. Regardless, whatever money was in your account at the time that stolen money came in you are entitled to it back.
Make a formal complaint to the bank and if they still will not refund you you can either complain to the Financial Ombudsman Service issue Small Claims Court Proceedings www.moneyclaim.gov.uk or actually do both.
Can I clarify anything else for you?
Please rate the service positive. It is an important part of the process by which experts get paid. It does not cost you anything but helps us greatly.