ok I just had the same instruction landing into Cardiff today for first time; did my head in! Was approaching from North East for runway 30. ATC said right base for 30. I'm thinking ok so I should be turning right into the base leg but then I'm thinking that can't be correct because in a right hand pattern for 30, to turn right I would have to make a right turn from downwind leg conflicting with crosswind traffic!
So while deliberating and aiming down left side of airfield, suddently resalised there was another runway to my left. Checked with ATC and found I'd lined up for St Athan! So ATC suggeted a quick 180 that put me downwind in RH cicuit for Cardiff. Phew! Landing was spot on though, one of my best!
So I guess when joining right base when coming kind of straight in to the runway direction means turn left onto base!