Thanks anyway!
I did a system reset after talking to Pioneer yesterday at lunchtime. It crashed once on the way home, but has behaved otherwise. Pioneer said if that didn't work, to remove the battery power from the unit for a minute. If that doesn't fix it, they said that I would need to send it in with $230 for repair. Not going to be happy if it comes to that as the unit is only 2 1/2 years old and has been well cared for. I'm not sure if the problem is a fault of the unit, or a problem with Apple CarPlay.