That is awesome, going to look fantastic once it is all painted up. Space Hulk is a great game - another example of why GW should stick to making boxed, stand alone games that they actually do well and leave the wargames rules alone.
I have done a castle with them too but that was over 7 yrs agio now and is long gone. Might have some pix on a old computer somewhere. I've make almost all of Hirst items from his molds. even came up with a lot of my own. Use to cast them in smaller number kts and resin cast them but that was yrs ago too. pix of a set I made when I had my store. Use to break the box sets down and add to my sets.
I've often looked at those Hirst Blocks as well. As with Thansants its a problem of where to stop when it comes to buying them. Of course if we were clever we'd each buy some and swap either the moulds or blocks. But that would require a level of thought & organisation beyond the abilities of most of us this early in the morning.
Beautiful work by the way - probably the first large scale terrain project I dreamed up as a teenager was a 3D Space Hulk board, but then I discovered booze & cigarettes....