ok found this video of someone using another UI ( the name escapes me at the moment ) and shows everything working as expected.
http://www.youtube.com/watch?v=uxn4zL-094M
The only difference with this fight compared to any others is that the special bar is being displayed all the time and not just popping up as and when you need to click it. Whether this particular UI is forcing it to be displayed all the time I don't know. Hence the need to see how blizz is handling this so that we know what to ask blizz to tell us so that we can react similarly.
I really can't understand why just on this fight ( at present ) this is happening but all other times ( so far ) it is working properly.