Graham just seen your message not sure if you've had any answers.  Firstly let me say you've picked a really good place to stay, the Hermes is great and so friendly, as are Diana and Sabastian the owners and their family.  I'll be going in September for the fourth time and my parents the eighth!!  The Hermes is full of people who go back year in year out.  Back to your question, there is a hole in the wall up in the old town, its a bit of a walk up though and there's always the chance thats its out of action.  There are safety deposit boxes in the hotel reception though and Diana will change travellers cheques for you so you don't have to go down to the town to change any.