Having been lucky not to have encountered bracken fern, can not say what will be good for it.
It's ineffectiveness so far may lay in the fact that Ferns take up nutrients and water very slowly.
If i said what to use I would be making it up.
Sorry you just found my Kryptonite
What you can be assured of is that adding seasol to whatever herbicide is effective will "amplify" it's effect just as well.
Thanks Fu, the deed is done...