Hi @Gary_Mortimer,
I’ve used a Lua script for this type of steering in ArduPilot:
Enable Pivot Turn for Custom Rover Frame (there are scripts attached to the topic, they could do with an update)
It’s a generalised version of the kinematic bicycle steering model. I’ve run it on simulations in Gazebo and an RL rover. Let me know if you’d like this added to ArduPilot as a PR and I can prepare something.