I know it must be a pain as a developer, but the best way to stop people cracking your demo is literally to have two binaries

I have done that but I'm not sure it's really the best way. My stats are very far from being statistically significant but I'd say this type of protection has increased the fraudulent (stolen credit card) orders about tenfold for me. Which means that if I don't catch it on time I'm hit with $15 chargeback fee. It's really frustrating because I've deliberately made the trial version very easy to crack. I have no problems with cracked copies floating around, I just didn't want serial codes being freely available.