No longer able to run Xamarin Forms iOS app

Morning

We have a Xamarin Form app which we're able to build and run on Android but not iOS. The app will compile, install on the simulator, and we get the launch image, but as soon as we hit Forms.Init() in the AppDelegate.FinishedLaunching we get an unhandled exception:

Unhandled Exception: System.IO.FileNotFoundException: Could not load file or assembly 'Mono.Android, Version=0.0.0.0, Culture=neutral, PublicKeyToken=84e04ff9cfb79065' or one of its dependencies.

I've cleaned the solution, cleared packages, checked referenced, checked the .csproj and I'm not able to see any reason it would be trying to load Mono.Android files on launch.

VS 2017 Enteprrise 15.7.3, Xamarin 4.10.10.1, Xamarin.Android SDK 8.3.3.2, Xamarin.iOS And Xamarin.Mac SDK 11.12.0.1

Any ideas?