@Clive Wickham
Apologies on delay on this, I was checking it internally.
This article shows one way to expose a static, public IP address for a container group by using an Azure application gateway. Follow these steps when you need a static entry point for an external-facing containerized app that runs in Azure Container Instances.
As long as the application gateway runs and the container group exposes a stable private IP address in the network's delegated subnet, the container group is accessible at this public IP address. Please check if this is something that might help your scenario.
I am checking with ACI product group to see if we have any ETA for - Windows container in ACI support virtual network and multiple-container groups in the near future. I will share it here when I have an update.
Let me know if you have further questions.