Hi
I'm going to answer my own question here - the car's BT supports phone but not music (that came with a later model). It occurred to me to contact a car stereo system installer, and that's what I was advised.
I did try connecting my phone's headset jack to the car's front aux socket with a 3.5 mm audio lead, paired the two devices, told the phone to play a particular playlist, and it did, over the car speakers. The reproduction quality was good, but I had to crank the car's audio volume right up (it seems the car speakers were merely amplifying the relatively weak phone output). It automatically paused/disconnected when I told the phone to make a phone call, so I seem to have the functionality of both phone and music that I wanted.
This may be a common workaround for cars with limited BT capability, but I'm pretty happy with the outcome of these efforts.
Cheers