im trying to assist my mother in changing the name on my parents property and bank accounts in Spain following the death of my father. They have a Spanish will, the property and bank account are in both their names,
You don’t say whether your mother and father are/were permanently resident in Spain or elsewhere, nor what the terms of your father’s Spanish will are. (‘They’ can’t have a Spanish will, or an English will for that matter.) I imagine that they each have a Spanish will and it’s possible that the terms of these leave their share of the Spanish assets to the remaining spouse (or to you and any siblings in the case of the second death.)
There are, however, plenty of other possibilities, such as passing ownership of 50% of the house to you (+ siblings) whilst leaving the surviving spouse (your mother, in this case) with a life interest in the property. (This is sometimes done to avoid two lots of inheritance tax.)
What I’m trying to say is that this is not necessarily as straightforward as it would be in the UK, and that professional help is probably going to be needed, This shouldn’t be madly expensive, but I’d suggest contacting whoever drew up the will(s) and ask them to give you an estimate of costs and of any Spanish taxes that might be payable.