Just a followup, for anyone vaguely interested.
Seems android and bluetooth are a right old state. On the basis that my useless Windows Phone's voice control works flawlessly, the android state of affairs is positively cringeworthy.
I have tried out about 15 apps, which claim to be able to drive the audio through the bluetooth. None did. A lot of googling reveals that android seems to want the A2DP device profile in it's headsets to get audio (as opposed to phone calls) to them. I would accept this, if it wasn't for the fact that the headset I use with my Windows Phones doesn't have A2DP. Yet it picks up the phones audio (certainly the voice prompts).
Finally I stumbled on "Voice Control" - a paid for app, which has a test version available, so you can test it works with your setup before you buy. I have set it up for the Mrs to try. Early signs are promising, although it's a battery hog.
Funny - I'd happily shell out £10, maybe £20 for a tool which looks like it *might* be useful. But £1.29 for an app - oh no, I have to trial it first !