Strange... I just went out to test and it connected first time and stayed connected. I switched off the ignition about 10 times, each time waiting for the bluetooth connection to disappear from the phone, and each time it reconnected perfectly.
Guess I'll watch it over the next few days and see how it goes. It's booked in Friday for service anyway so worst case I'll ask them to check it out.
Thanks all for your suggestions, etc.
Cheers,
Tom