Thanks Mike - David says minimum power supply for an Opteron 175 dual should be 520W - this may explain why it sometimes starts and other times not. Looks like you may need a new power supply? He says as long as it is AMD approved 500-550W should cover it.
Hope this helps
Andy

Thanks Andy, I am sure that is it, makes sense. More expense!

Well fitted a Cool Master 520W PSU which can peak to 600W and guess what - no difference! At least the computer seems to boot into dual mode from a cold boot but always single mode from a restart. If I have to restart I just turn it right off, wait a few seconds and boot and it works. I will just have to live with it but thanks for your help Andy.