Very sorry to hear of your loss
Being named as an executor in a will in fact has no legal standing until you obtain probate, which is the first thing the executor normally does. This means getting forms from your local probate office, completing and returning them. Once you have probate issued, you should find dealing with banks and building societies far easier and straightforward.
On the other hand, if the affairs of the deceased are relatively simple, then you may prefer to avoid going through this process. If you don't have probate however, I'm afriad that some financial organizations can be rather difficult and time consuming in making you prove your entitlement.