Thank you (I am a bit irritated to realise that I did not put this thread in the PSU section).
I have just had a look at my BIOS and C7 sleep option is squirreled away in a CPU section that I had never seen before (it was off).
It really isn't clear but, as EsaT says, my PSU does seem to be likely to be DC-DC which, apparently, is not affected by the C7 sleep state.
I have turned it on (changed it to Auto), to see what happens.
What is C7 sleep, anyway?
Is this when the system goes into hibernation?