Graeme,
The higher the better, but you do have to use a bit of
common sense and 'balance' about the whole thing. (Common
sense meaning 'frugal', 'cheap', 'penny pinching', 'lets
not get carried away whith this thing!') Unless they build
chimneys a lot diffrent there than they do here, the average
chimney isn't designed to support the weight and wind load
of an antenna. The difference between a few feet doesn't
really make that much difference in what you will hear. A
few tens of feet, yes. A few feet, no.
SWR is the measure of how well the antenna impedance is
matched to the feed line's impedance and how well both of
those are matched to the transmitter's output impedance. An
antenna will have to be tuned no matter where you put it,
unless you're extremely lucky to have a good match to start
with (if you're that lucky, play the lottery!).
You might take another look to see where else you can mount
your antenna that wouldn't envolve the pitter patter of chemney
bricks hitting the roof...
'Doc