This does not sound like a power button issue since the PC seems to proceed through the power on self-test (POST) without a problem. My guess is that there may be a problem with your hard drive since Windows does not seem to load. I recommend that you backup your data and perform some sort of hard drive diagnostic. Here is some simple tests: https://www.disk-partition.com/windows-10/hard-drive-test-windows-10-4125.html

I used the first method ie using the Windows tools and no errors were found on the drives. On my pc there is a 32Gb ssd cache.

What happens when Windows does not load/start? What is displayed on the screen, does the spinning dots continue to spin or are they frozen?

The dots don't appear. The Dell logo remains on the screen. The lights go out on the mouse and keyboard.

I the 32Gb cache just used at startup? If so is it practical to disable it. I only start up once a day so the faster start up is not critical.

If the spinning dots do not appear it means that Windows has not started to load. The cache is not just used at startup. There is a feature in Windows 10 called Fast Startup that may be the problem. In Fast Startup, the state of the system is saved in the hibernation file. When the computer is started again, the system is reloaded into RAM from the hibernation file. You can disable Fast Startup by going to All settings / Power & sleep / 'Additional power settings' (on the right) / 'Choose what the power button do' / uncheck 'Turn on fast startup' under Shutdown settings. You can read more about Fast Startup here: https://www.howtogeek.com/243901/the-pros-and-cons-of-windows-10s-fast-startup-mode/

thank you all   for your helpful comments. Will explore FastStartup.

three days later and my once daily starts have been fine - no problems - and an insignificant amount of  extra  time to start

thanks for all the advice.

So, it appears that disabling Fast Startup solved the problem. Is that what you did?

Windows crashing before it finishes.  This is a bad sign indicating hard drive may fail at any time.

Should do a full backup and reinstall on a new hard drive.

The cause of these errors depends on the specific error but is usually related to a problem with a device driver, firmware, or hardware failure of a storage device.

Log Name: System
Source: disk
Date: 10/13/2019 6:31:28 AM
Event ID: 52
Task Category: None
Level: Warning
Keywords: Classic
User: N/A
Computer: Media
Description:
The driver has detected that device \Device\Harddisk0\DR0 has predicted that it will fail. Immediately back up your data and replace your hard disk drive. A failure may be imminent.