Wayland - it is indeed an 'uphill battle'. My wife used to work for a conservation charity, and was involved in the fight to save West Thorrock Marshes. Although a power station had been built on what were once grazing marshes (which sounds very unnatural). However, as the power station closed down, nature moved in, and its become home to 1,200 species of bug, bird and reptile, many of them extremely rare.
Of course someone wanted to build on it (the Royal Mail), and someone wanted to let them (the local Development Corporation - which as such, had extraordinary powers). The charity (and others) did their best to fight the plan over three years, with very limited resources, and almost won. The problem is that brownfield legally means just that. Its former industrial land, and thus regarding as having no natural value, even though this site is probably the second most important inverterbrete site in the UK.
Brownfield sites basically have no protection from development, no matter what wildlife is on them. You can have more protection on a piece of 'countryside', with far less biodiversity on it, simply because the latter is 'countryside'. And of course a local authority will vote in favour of development not just for the usual economic reasons, but also because they are working to national targets for building. And they may actaully own the site...
Thats not to say you should never build on brownfield, and its certainly preferable to building on a greenfield site, which will never been greenfield again, but there should be the same level of protection for biodiversity and important habitants that there would be for a greenfield site.