Seren, I don't know if you're still thinking of Bar Cwtch in The Bay, but apparently it's closed. There's another one opened in Cathedral Road in the Jolyons Hotel.
I did a quick ask around in work today and The Cosy Bar was named as a good place where they could corner of a section for us and do a buffet. Also Tiger Tiger. I've NO clue where abouts these are but I think they are pretty central.
Other options if you prefer Cardiff Bay would be The Salt Bar or... immmm... No it's gone. It may come back to me