I know! I have never experienced this with a MB before ever! Its weird but hey there it is, system working fine even with that 420w power supply in the standard case, but that said, this is a bare basics system, no graphics cards no add ons only running basic email office programs network printers so nothing much attached.
Thanks for all your pointers anyway, and glad to provide my little experience as useful info tucked away for a baffling moment.
R