It�s a shame your late husband didn�t leave this information easily accessible for you or better still take out the mortgage in joint names.
As his Executor you should have been able to register the death immediately although without Probate you may not have been able to take over repayments. This should have alerted the Bank to their procedures for securing the repayment of a loan from an Estate rather than an individual so that you had the full details of the debt to declare in order to get Probate.
Have you looked at the terms of the loan? It will usually say in the small print a) what happens when the mortgagee dies and b) how to complain.
I recommend you write to them in the first instance using their complaints procedure (all banks have one � if you can�t find it easily on their website ask at a branch) and asking for a return of charges incurred since the date you registered your late husband�s death. If you get no joy from the internal procedure go to the Ombudsman.
Good luck.