Reporting back...
I tried the Super Bock and Sagres lagers, available everywhere. Also found some of their different beers in a nearby supermarket - Sagres Bohemia and Preta probably the best of the bunch (but still didn't entice me to buy more of them). Super Bock Stout was the first 'stout' I've tasted with sugar and caramel colouring in it!
I did stumble upon a bar just off the square of the Old Town called 'The Beer Spot'. It has a decent selection of Trappist and Abbey beers as well as a few other decent beers and was certainly a nice spot to spend a few hours. If visiting the area, it's worth a visit.