Thank you Ralph.
I have tried starting with the optical drive removed. No joy.
I will try and remove the memory sticks as I still have the smaller ram sticks which came with the machine originally.
With regard to the 'system disk', are you referring to my OS X install disk? Should I try booting (holding down key C at startup) to boot from the disk in the optical drive?
Please excuse my ignorance, I am still rather new to Macs; having the Pismo has taught me a great deal in a very short time.
Mal