[Steve_Ray]

It's cell (and table) margins you are looking for, I think. In the tale
properties dialog (table properties table or cell tab options), you can
set inside margins.

Thanks for your reply. Changing those doesn't appear to have any effect.
The L and R cell margins were 'zero' anyway, and the T and B were 0.08".
When I reduced the latter to zero, there was no visible effect.

So I still can't see what's controlling that 'white space' in the cells' T
and B margins (which border the text 'as if' they were instead L and R
margins since the 'text direction' in those header row cells is vertical).