iPhone 12 connects fine to my 67 plate Amunsden, but I cannot control the phone from the steering wheel. The phone button does nothing. As soon as CarPlay is disconnected I can control the phone via Bluetooth as normal. Is this correct.

I think so. The idea is you use Siri to control the phone. The button should work for answering and ending calls though. 
 

I think that’s correct anyway. I rarely make calls in the car so memory is a little fuzzy. 

There may be a better method but I use Siri to control the phone. If you press the voice control on the steering wheel quickly then the standard Skoda voice system kicks in. If you press and hold the voice control button, it activates Siri. That’s how I make my phone calls. 

Thanks for the info, just seems odd that it appears to be a retrograde step from using the scroll wheel and the phone button just to be able to use CarPlay. Voice control will not be as effective or accurate as physical buttons so seems pointless having CarPlay on a car with SatNav and integrated Bluetooth. Would be great to be able to disable the phone element of CarPlay but that’s not an option.