I want to create a pivot table whereby I can pivot around date, but
only display result within a certain time period, such as between 6am
and 6pm (yes, there's lots of other data which I can display just
fine)...

Actually, I want to display through from 6am on one date through to
6am on the next (my business "changes date" at 6am).

Is there a way I can do this? I can't find a way to easily filter by
time on a pivot table (trying to use it as a page field doesn't work -
I can't set up an automatic filter like this).

Any ideas?