Custom Bindable picker selectedItem not firing

I have a custom BindablePicker which inherits from picker, having copied some code in a previous post. However I am finding that when an item is selected, my property is not being updated.

BINDABLEPICKER
public class BindablePicker : Picker
{
#region Fields

        //Bindable property for the items source
        public static readonly BindableProperty ItemsSourceProperty =
            BindableProperty.Create<BindablePicker, IEnumerable>(p => p.ItemsSource, null, propertyChanged: OnItemsSourcePropertyChanged);

        //Bindable property for the selected item
        //public static readonly BindableProperty SelectedItemProperty =
        //  BindableProperty.Create<BindablePicker, object>(p => p.SelectedItem, null, BindingMode.TwoWay, propertyChanged: OnSelectedItemPropertyChanged);

        public static readonly BindableProperty SelectedItemProperty = BindableProperty.Create("SelectedItem", typeof(object), typeof(BindablePicker), null, BindingMode.TwoWay, null, propertyChanged:OnSelectedItemPropertyChanged);

        #endregion

        #region Properties

        /// <summary>
        /// Gets or sets the items source.
        /// </summary>
        /// <value>
        /// The items source.
        /// </value>
        public IEnumerable ItemsSource
        {
            get { return (IEnumerable)GetValue(ItemsSourceProperty); }
            set { SetValue(ItemsSourceProperty, value); }
        }

        /// <summary>
        /// Gets or sets the selected item.
        /// </summary>
        /// <value>
        /// The selected item.
        /// </value>
        public object SelectedItem
        {
            get { return GetValue(SelectedItemProperty); }
            set { SetValue(SelectedItemProperty, value); }
        }

        #endregion

        #region Methods

        /// <summary>
        /// Called when [items source property changed].
        /// </summary>
        /// <param name="bindable">The bindable.</param>
        /// <param name="value">The value.</param>
        /// <param name="newValue">The new value.</param>
        private static void OnItemsSourcePropertyChanged(BindableObject bindable, IEnumerable value, IEnumerable newValue)
        {
            var picker = (BindablePicker)bindable;
            var notifyCollection = newValue as INotifyCollectionChanged;
            if (notifyCollection != null)
            {
                notifyCollection.CollectionChanged += (sender, args) =>
                {
                    if (args.NewItems != null)
                    {
                        foreach (var newItem in args.NewItems)
                        {
                            picker.Items.Add((newItem ?? "").ToString());
                        }
                    }
                    if (args.OldItems != null)
                    {
                        foreach (var oldItem in args.OldItems)
                        {
                            picker.Items.Remove((oldItem ?? "").ToString());
                        }
                    }
                };
            }

            if (newValue == null)
                return;

            picker.Items.Clear();

            foreach (var item in newValue)
                picker.Items.Add((item ?? "").ToString());
        }

        /// <summary>
        /// Called when [selected item property changed].
        /// </summary>
        /// <param name="bindable">The bindable.</param>
        /// <param name="value">The value.</param>
        /// <param name="newValue">The new value.</param>
        private static void OnSelectedItemPropertyChanged(BindableObject bindable, object value, object newValue)
        {
            var picker = (BindablePicker)bindable;
            if (picker.ItemsSource != null)
                picker.SelectedIndex = picker.ItemsSource.IndexOf(picker.SelectedItem);
        }



        #endregion
    }
}

VIEW
<local:BindablePicker Title="Relationship" ItemsSource="{Binding PersonLinkTypes}" SelectedItem="{Binding SelectedLinkType}"/>

VIEWMODEL

            private string _selectedLinkType;
                public string SelectedLinkType 
                { 
                    get
                    {
                        return _selectedLinkType;
                    }
                    set
                    {
                        _selectedLinkType = value;
                    }
                }

        private IEnumerable<string> _PersonLinkTypes;
        public IEnumerable<string> PersonLinkTypes 
        { 
            get
            {
                return _PersonLinkTypes;
            }
            set
            {
                _PersonLinkTypes = value;
                OnPropertyChanged("PersonLinkTypes");
            }
        }