The bats don't like having a light shining on their resting place (thanks, Pea!), so have now moved to a tree in a dark corner of the garden, which also has some berries. This means we'll have to get a separate spotlight and shine it up into that corner.
The problem may not be as bad as we thought though, as the bats are not roosting there, which is where they stay in the daytime, and are only feeding off the temporary supply of berries. So when that supply dries up, hopefully they will leave.
One neat thing we discovered though is that shining our veranda light out into the garden makes it look really nice! Well, it looks nicer than my photo shows, but you get the idea. While I was out there taking photos I thought I'd go on bat patrol. I used the flash in my camera to flush out any bats in the dark corner they've moved to (there was one). The resulting photo shows the blue berries (at the right, click to enlarge) they are attracted to. I have no idea what the name of the tree is, and unfortunately it is right near my clothes hoist, which can be seen at the bottom of the photo. So far no horrid purple stains have landed on any washing.