Update on the cracked tank: I got my refund and was about to take the tank apart when I decided I should give a local glazier a call and ask about a piece of float glass to completely replace the broken back pane. I got a piece cut, and I had the old piece cut down to make a partial cover for the tank (I removed it using dental floss to cut the silicone and it was pretty easy.) All for £16. So, when my £4 worth of silicone arrives I'll find out if I can make a Signature 600 for £20.
Not sure where I'll put it though, since in the meantime my girlfriend and I decided it was too large for that room anyway and I ordered a Dennerle 60l cube instead. Currently I'm playing with the idea of making a nano-reef from this and replacing Crypts Palace with the Signature (which might require me to reinforce the table it sits on).