Hi Peter. (Fanboy Gush: stunning software Sir!)
I'd tried to route via the control panel. I've got three AirPlay devices on the network and have tried them all individually (and together). It doesn't matter if they're selected, I'm afraid the sound just outputs from the iPad speaker. However, as I mentioned previously, if a Bluetooth speaker is paired the sound diverts to it as it should.
I couldn't find this issue mentioned anywhere else on the support site - or on the internet - so maybe it's just a setting in my device somewhere. If you've got any other suggestions I'd appreciate them.