No, I didn't think there was a direct way of doing this. The problem with multiplying by 5/7 is that while it's fairly good for long durations, it doesn't work well for short. An event that start on the Friday, for example, and finishes on the following Monday, would have a working duration of one day: by multiplying 3 calendar days (Saturday, Sunday, Monday) by 5/7 would give, er, 2.14.
The annoying thing is that I have worked this out in the past, using syntax, so I know there's a solution out there somewhere!
Anyway, thanks for your help. Will keep trying.