I was overlooking the Progressive Beer Duty that some of the regional brewers keep on whingeing about (why don't they just stop supplying the supermarkets?).
I don't know the exact details but I believe the full 50% discount only applies to breweries producing less than 5,000 hectolitres per annum which if I've calculated correctly is just under 400 firkins. I would have thought that they brew a lot more than this. I still agree that it's a rip-off irrespective of how much they brew.
EDIT: I have checked my calculation and think that should be just under 4,000 firkins!
Given that Dark Star have two tied houses and are seen as guests all over the place I would think they must easily exceed that figure.