Spurred on by the solstice (cloudy in southern England though I enjoyed the dusk falling over the sky anyway), I found a couple more sites online that look more helpful. I think the trick will be to get a sunrise time and plug that into the azel.html page, then give it say 10 minutes after (or whatever will safey mean the sun is above the hilltop of interest), and plug that in too. Now you have two azimuths - is that hill in between them or totally out of the question? Still quite time-consuming.

http://www.srrb.noaa.gov/highlights/sunrise/sunrise.html
http://www.srrb.noaa.gov/highlights/sunrise/azel.html

But I've tried the Excel file of the same NOAA calculations at http://www.ecy.wa.gov/programs/eap/models/twilight.zip
and it's just as good, and you don't have to be online for it.