Have a look in the RSS feed from the BBC weather centre:
wget -q -O - http://feeds.bbc.co.uk/weather/feeds/rss/5day/id/1671.xml \
| sed -e '/Sunset/!d'|sed -e 's/^.*Sunset: \(.*\)GMT.*$/\1/' | head -1
Replace 1671.xml with whereever you are.
Have a look in the RSS feed from the BBC weather centre:
wget -q -O - http://feeds.bbc.co.uk/weather/feeds/rss/5day/id/1671.xml \
| sed -e '/Sunset/!d'|sed -e 's/^.*Sunset: \(.*\)GMT.*$/\1/' | head -1
Replace 1671.xml with whereever you are.