The solution offered ( reset the CMOS ) did not fix the issue, HOWEVER, it got me to go back in once again into the BIOS and look at settings.
I have now resolved the issue.
Turbo Mode stays enabled if, in addition to:
PCIe performance mode to OFF
CECP to OFF
Turbo Mode: ON
I ALSO set:
Runtime Power Managent to ENABLED
My Z420 is now happily enabling turbo when it needs it , my 3.2GHz processor happily goes up to 3.8GHz with 1 core heavily used and 3.5GHz with all six cores heavily used.