OK thank you, I don't know exactly what happened, I closed Xamarin Studio, reopened, cleaned the project again (I had already done this) rebuilt and the app started working on iPad, so I guess just one of those Xamarin .NET things. This has happened to me in the past with VS2015 where a project started playing up and required all packages to be removed and re-added to fix it.
All good now, thank you, and great control