Bracken is a difficult one to get rid of and most farmers spray it in large areas. I would ask at your local Agri merchants or get the phone number for Dow Chemicals who should be able to advise. In the meantime, can you electric fence the area off?
I think you can spray or wipe bracken with round up (definitely know thats round-up does the horrid balsam!). We have a little bit of bracken - but the horses never eat it as too much grass in a huge field so don't really worry about it too much
Bracken is very tough and is difficult to get rid of. Glyphosate (Round-up is one product) will kill it when made to the highest strength. Be aware though that it will also kill grass and anything else it touches.
Use roundup, if its a large patch spray off the lot and throw down some grass seed, as the bracken dies the seed wll be growing and roundup will not kill the seed. Leave the leaves in place until completely dead to make sure the plant is dead to the roots..
if you have separate bits mix the roundup and add wall paper paste then paint it on the leaves, it will then only kill the one plant.