This is an excellent example of the reason for never ever appointing a bank as executor of a will! The proposed charge seems monstrous - any ethical body would make a charge based on the amount of work involved and not a scale charge based on the value of the estate.
However it is unlikely they could be removed, as they are not going to pass up the opportunity to earn some easy money! I think the only way would be to get a Court to agree they should be replaced, but this would cost in solicitors fees and time, with no guarantee of success.
An alternative would be to go through the bank's complaints procedure about them overcharging and then to the banking ombudsman if necessary. But if you do this now it will hold up the administration and distribution of the estate, so it might be best to do it after things have been settled.