River taff leader strength?

[Occoboy]

Firstly I know you all talk in 6X gibberish, but I'm quite new to fly fishing and I prefer to buy spools of fluorocarbon instead of special tippets/leaders. So I was wondering, does anyone know what bs/strength my leader should be to fish the river Taff in Pontypridd? Apparently the fish are mostly up to 1 to 1 1/2lbs.
However I've also seen people saying they regularly catch 3 to 4 lbers! But I'm not sure if this is furher down the river. :S
Any help would much appreciated. Thanks in advance.