I'm sorry to keep answering my own questions, but here we go anyway...
I'd assumed that because the line spacing box was reporting 12pt when I
selected "exactly" then that must be what the document was already set to -
in other words, I assumed that 12pt equals 1.5 spacing.
Now of course I've realised that it doesn't. I suspect that in fact 12pt
equals *single* line spacing (which is why reducing that figure produces
single line spacing, whatever the value). And I further suspect that 12pt
is in the box merely as a starting value, not as a reflection of the
existing document.
So - and I'm off to experiment with this - maybe *18pt* equals 1.5 spacing,
and therefore 17pt might give me the just-a-fraction-less format that I'm
after.
If all this is nonsense, somebody please tell me...
Bert
www.bertcoules.co.uk