Hi Steve
All the salsa nights in Cardiff run classes on the drop-in principle ie, you just go along and join in the class that's at your level - the cost is around the £3 mark - you don't have to sign up for a course of classes.
Some places can get a bit crowded and hectic, I would say the most spacious and relaxed places for a beginner to go would be Mondays at Life and Wednesdays at The Music Room (O'Neill's). Details on the
listings page.