Thanks to each of you for your responses. The collective wisdom and helpfulness within this forum continues to make it one of the most informative I read - and the most friendly.
All contributions are helpful, and whether hasty or no, it seems to me that the explanation by Bushfiva is the most probable one. I haven't had to study the details about the various power connectors and distribution systems in depth before and this has been educative as well as helpful.
Thanks for the tip about the pin 8 check BTW - that will be followed up when I next have the lid off my 'test' computer which hosts the new PSU,. and also for the reminder about the P4 plug - yes that one was dutifully attended to on each attempt.
best regards,
FOR