DuaneB:
Not sure why only 3 out of 6 worked, but happy enough for now and will have another go at the other three soon.
I'm not sure about the ATmega8 but in the 168 and 328 models there is a fuse in the Fuse High Byte that can be set to disable serial (ISP) programming. To get around that you would need a High Voltage Parallel Programmer (like the HV Rescue Shield).
Another possibility is that some really strange clock option has been set. Perhaps someone selected "External Clock". Or possibly the 128 KHz RC oscillator. That would require an ISP speed of 64 kHz or slower but on a 16 MHz Arduino the hardware SPI speed only goes down to 125 kHz. 