When I first spotted it, the spider was stationary in the middle of its web.
Then a small insect flew into the web, causing it to vibrate - and alert the waiting spider, which immediately rushed off towards the unfortunate victim.
Within a few seconds the spider was tucking into its meal - an aphid by the looks of it.
I wasn't specifically trying to photograph a spider. I just happened to see it as I was lining up a shot of my olive tree, whose fruits are forming now:
Don't get too excited. That olive is about 5mm in diameter!