WPF - Binding to current item from within group header style

I'm something of a WPF noob so please take it easy on me ;-)

I am trying to create a grouped DataGrid (WPF toolkit version). I have successfully created the data source, the DataGrid itself, the required CollectionViewSource and the Style for the group header (which uses an expander).

I want to group by a property called 'Assign_To' and have the relevant value (the value that the grouped items share) show up in the header. However, I cannot work out how to bind to the current group/item in order to return its Assign_To property.

The closest I have got (shown below) is binding to the overall CollectionViewSource, which returns a fixed value for Assign_To. What would be the proper way to bind to the current item/group in order to return the correct value for 'Assign_To'?

Hope someone can help. Thanks!

Andy T.

Here's the source...

<Window DataContext="{Binding Source={StaticResource SampleDataSource}}"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:dg="http://schemas.microsoft.com/wpf/2008/toolkit"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    x:Class="DataGridTest.MainWindow"
    x:Name="Window"
    Title="MainWindow"
    Width="640" Height="480" mc:Ignorable="d">

    <Window.Resources>
        <CollectionViewSource x:Key="CVS" Source="{Binding MyData}">
            <CollectionViewSource.GroupDescriptions>
                <PropertyGroupDescription PropertyName="Assign_To"/>
            </CollectionViewSource.GroupDescriptions>
        </CollectionViewSource>

        <Style x:Key="GroupHeaderStyle" TargetType="{x:Type GroupItem}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:T