Yippee! Turns out I had the CPU multiplier all wrong. I took the FSB figure of 266Mhz from the manual not realising that it really only runs at 133Mhz. I also managed to find out what clock speed the AMD Athlon 1800 XP actually runs at - 1533Mhz. So instead of the multiplier of 5.5 that I had, I needed 11.5. Now where did I put those OS disks? They're somewhere in amongst all this packaging and old PC bits.