Problem solved and sheepishly I have to admit it was answered in the second reply with the web site for shut down problems.
For anyone interested it was resolved by the bios screen options "power management" and selecting "fail safe ddefaults", nothing changes on the page but behind the scenes something obviously does as it now shuts down.
For completeness selecting optimal settings brings the problem back and downloading new flash bios from gigabyte web site and installing does not change this.
Thanks to everyone who helped.
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