After @Yeastface's post, I re-read yours and (what I believe YF was getting at) the way you capitalise CHATEAU and CASTLE appears as if you are trying to say that you bought Chateau but were given Castle. Now you've been replying to the thread it appears this isn't the case so I apologise.
However, I still don't see how them buying in bulk and selling in smaller quantities isn't a plausible answer.
Firstly, the best way to keep prices down (and GEB are one of the, if not THE, cheapest HBS around) is to buy in bulk quantities.
Secondly, how do you know that maltsters only sell their malt in 25kg bags? If Castle are selling wholesale to microbreweries and larger HBS's it would seem to be an unnecessary cost to bag everything up in 25kg bags, especially for base malts (although it wasn't a base malt you bought), where a microbrewery would be needing hundreds of kilos at a time or an HBS would be needing to crush the malt themselves (again I realise this wasn't the case in your situation)!
Finally, what you are trying to say they are doing (i.e. Passing off an "inferior" malt as Chateau Special B) is illegal and if you are so confident that this is what they are doing you should be contacting Belfast Trading Standards about it. They would investigate and take action if necessary. However, I would suspect they are answering truthfully considering their reputation!