Thanks for the advice; my friendly Computer expert has looked at everything and detemined that the power supply (the KGuard supply and the new one that I brought from Maplin) just wasn't 'man enough' (their words, not mine). I've brought another power supply (3 amp) quite cheaply, and it's all up and running. I'm still puzzled as to why the KGuard plug wasn't working correctly; do they lose their power/influence? I don't know.
If anybody else has similar problems, please strongly consider the power supply no matter how new it might be, and no matter whether or not it came in the original package.
Thanks again,
Chris W.