hmmm... ok. I have no idea what the issue is then. Aside from the digital version which has been tested extensively on various emulators, the game has been released as a physical tape so there are quite a few people out there playing on real hardware and I've never had an issue. It's very odd.
So I built a Spectrum Issue 3b from scratch using all new parts, it works perfectly. But this game crashes in exactly the same way as on the original Issue 2. So the only thing in common hardware wise is the original (1980s) Ram Turbo Joystick interface... unplug that and the game works fine. Maybe some bug in the joystick controller?
right, I think I understand the issue - I'm afraid the game is not compatible with certain old joystick interfaces which don't actually follow the correct protocols and cause an issue with the interrupt. In the 80s, coders would create a 257 byte lookup table to account for this issue as it was a fairly common issue, but this is the first time I've had anyone have this issue.