Thanks everyone for the suggestions so far, some really good ideas. Swyd yr Eira does seem great and of course the other waterfalls in the area. Hard to get much closer than right behind it!
I like the cave idea too if we want some weirder effects.
High Force seems a good option too. Looks like a huge amount of water going over, can imagine it's very loud. Does anyone know how close you can get?
I agree most waterfalls in some state of spate would really suit the bill, though some you really do have to shout when you're up close and that's the kind of thing we're after.