It seems, as has been commented, I was over complicating this. It doesn't require a lambda at all. The following 'standard' C# syntax works fine:
Padding = new Thickness( 10.0, ( Device.RuntimePlatform == Device.iOS ) ? 20.0 : 0.0, 10.0, 5.0 );
I knew the question was probably stupidly simple. (Where's the thicko emoticon?)