Bracken - How to get rid of it?

So, I have bracken taking over my garden. I have half an acre of heather behind the house, and the bracken is moving in. It grows fast, blocks the light to the heather, then falls on it in autumn, smothering it and killing it off. No animals will eat bracken, as it's toxic. The spores are carcinogenic. The roots contain cyanide. All in all, not a nice plant. Roundup will kill the heather too... Apparently, bruising the bracken will gradually kill it off, so I've spent lots of time out walloping it with a stick. It must be having some effect, as the next field, "un-walloped", is now a forest of 6 - 8 foot tall bracken fronds. Apparently, a selective herbicide called Asulox (Bayer chemicals) will work to kill it off without harming the heather, but I don't seem to be able to get it anywhere. Anyone else have any ideas?