Not so much a bug, as slow!

[britlion]

As an addendum - if I ask it to calculate X^2 in the middle of the loop, it crashes the program. Calculating X*X works fine.

This is expected, unfortunately: ZX Basic uses the ROM, and a^b raises Invalid Argument if a < 0.

Wow. You're right. The ROM is completely wrong there. (-1 ^ 2 = 1). No, that's not your fault!