You wont have any problems using your Mastercard. There are a couple of ATM's in that area that accept all major credit cards.
We have always used our Nationwide Flex card in the ATM's in Pernera. There are a couple fairly close to each other at the bottom of the hill just below Knights Bar.
Always get a good exchange rate but new charges brought in at the end of last year may negate that now, but I would rather take my card than cash.