OK I've only looked at the system log. I'll see if there are any interesting power related items in the other log files. And system spec is now in my profile.

- - - Updated - - -

This error seems to come up in the system log at the time of almost all of the failed reboots;
The server {AB8902B4-09CA-4BB6-B78D-A8F59079A8D5} did not register with DCOM within the required timeout.
Nothing about power options, But I did find this in the log-files in the applications logs;

The following application attempted to veto the shutdown: Persbackup.exe.

Could this be it?
I've been using this s/w for years, but there have been various updated versions and the current one was 24th April - which must be around the time I started noticing this issue.