Thanks Rod - similar answer to that I got on ephotozine, and it's worked!
Don't use on camera flash often, but needed to prove a quick point to someone last week at a pre-wedding shoot. The bride and groom plan to get married standing just inside a barn door, with the guests outside - my immediate thought was urk, backlit. Luckily I had taken along an external flash so could show her what I meant.
Here's the link that was sent to me on the other forum: http://www.jakereinig.com/b/?p=130