How can I set a persistent bookmark, showing just the date and time
please? The date and time should be in the form of 'YYYY-MM-DD HH:MM' -
using the 24-hour clock system - and they should be persistent to
survive if/when emacs crashes please?

And following on from that request, how can I visually see the bookmark
set in the file, amongst all the text, please?

Its just that when I've been working on a very long document it would be
nice to go away from the document and know that I could return to the
same place if the computer went down with a powercut.

Thanks
Sharon.
-- 
A taste of linux = http://www.sharons.org.uk
TGmeds = http://www.tgmeds.org.uk
Debian 8.4, fluxbox 1.3.7, emacs 25.0.95