@j0hn:
Have eventually got back to normal Windows 10 again but only by constantly attempting starts & stopping, I think the situation with the F12 function key may hopefully become clearer in my reply to parkdale below.
After reading the above, are you using a wired or wireless keyboard?, if the latter get a wired keyboard and connect using the usb port (next to the ps/2 connector....not on newer Dell's!)
I have had issues in the past getting into Bios on some Pc's and it (nearly) always comes down to using a wired keyboard in the right usb port...
@parkdale:
Many thanks for your input most appreciated.
This is the latest Dell PC and does not have PS2 ports (as you said), has 4 USB-2 ports at the rear and 2 USB-3 ports at the front, the keyboard is wired, USB, as supplied.
I believe your comments are indeed relevent as by changing keyboard USB port I can now produce the "re-boot lock up situation" with F12 key as per any other attempt to execute startup functions so does now work as such.
I probably have not explained the anomaly with this PC very well so will try to clarify:
I could always access the startup options menu through Windows, holding shift & restarting or via "Setting/Recovery/Restart Now", however, any option chosen such as "boot to USB", "Advanced Options/UEFI (BIOS), Command Prompt" involve a restart to implement, this restart never completes, the PC does shutdown but remains frozen in that state until the power button is pressed which activates a complete shutdown.
The PC (usually) can then be restarted normally to Windows.
What I believe is happening, is that pressing F12 @ boot is now actually working but immediatly taking the PC to the "frozen state" as described above where only recovery option is power down & restart to Windows.
Very relieved that the PC is again back in a running state (especially as it's not my own) and will try to get some resolution from Dell support tomorrow.
Even though Windows is fully operational I am not at all happy with the situation as recover from any boot issue is currently impossible on this PC should the need ever arise, hopefully Dell will be able to provide a solution without the need to return the unit to base.