Ok, managed to get this all working.
You will need
Audi AMI composite AV cable with 3 RCA jacks (yellow, white, red)
Apple lightning to hdmi adaptor
usb powered hdmi to composite av converter (eBay)
interconnect cables (av and hdmi)
Connect it all together and viola! It works just fine.
The apple hdmi adaptor mirrors the iPhone screen (generally) so you can web browse, YouTube etc. It also will stream bbc news live, all of the catchup tv channels, Iplayer too. 4Od does not work however.
The apple hdmi adaptor also allows you to charge the iPhone at the same time by plugging in a lightning cable to it at the same time.
I do have an Apple TV at home so need to do some experimenting with that to make it far less wire heavy in the car.
just need to figure out how to power one without using an inverter!