>I think what is happening is that in using reflection to get the template, some of the bindings are getting evaluated and returning their value instead of the binding. Hence:
>
>IsChecked="{Binding Path=IsExpanded, RelativeSource={RelativeSource TemplatedParent}}"
>
>becomes:
>
>IsChecked="False"

After some digging I can see that you were right on both counts: GetMeTT was behaving as you suggested and the binding for IsExpanded should have been as you suggested.

I actually finally got a working template from Expression Blend. Interestingly, EB actually set the ControlTemplate for the ToggleButton in a seperate Style and used that in the TreeViewItem ControlTemplate. In case you are interested here's the EB version:

    <Style x:Key="TreeViewItemFocusVisual">
        <Setter Property="Control.Template">
            <Setter.Value>
                <ControlTemplate>
                    <Rectangle/>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
    <Style x:Key="ExpandCollapseToggleStyle" TargetType="{x:Type ToggleButton}"
           xmlns:Custom="http://schemas.microsoft.com/winfx/2006/xaml/composite-font">
        <Setter Property="Focusable" Value="False"/>
        <Setter Property="Width" Value="19"/>
        <Setter Property="Height" Value="13"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type ToggleButton}">
                    <!--<Border Width="19" Height="13" Background="Transparent" BorderThickness="1,1,1,1">-->
                        <Border SnapsToDevicePixels="true" Width="9" Height="9" BorderBrush="#FF7898B5" BorderThickness="1" CornerRadius="1">
                            <Border.Background>
                                <Custom:LinearGradientBrush EndPoint="1,1" StartPoint="0,0">
                                    <Custom:GradientStop Color="White" Offset=".2"/>
                                    <Custom:GradientStop Color="#FFC0B7A6" Offset="1"/>
                                </Custom:LinearGradientBrush>
                            </Border.Background>
                            <Path Fill="Black" Margin="1,1,1,1" x:Name="ExpandPath" Data="M 0 2 L 0 3 L 2 3 L 2 5 L 3 5 L 3 3 L 5 3 L 5 2 L 3 2 L 3 0 L 2 0 L 2 2 Z"/>
                        </Border>
                    <!--</Border>-->
                    <ControlTemplate.Triggers>
                        <Trigger Property="IsChecked" Value="True">
                            <Setter Property="Data" TargetName="ExpandPath" Value="M 0 2 L 0 3 L 5 3 L 5 2 Z"/>
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

    <Style x:Key="StdBase2" TargetType="{x:Type TreeViewItem}">
        <Setter Property="Background" Value="Transparent"/>
        <Setter Property="HorizontalContentAlignment" Value="{Binding Path=HorizontalContentAlignment, RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}"/>
        <Setter Property="VerticalContentAlignment" Value="{Binding Path=VerticalContentAlignment, RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}"/>
        <Setter Property="Padding" Value="1,0,0,0"/>
        <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
        <Setter Property="FocusVisualStyle" Value="{StaticResource TreeViewItemFocusVisual}"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type TreeViewItem}">
                    <Grid>
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition MinWidth="19" Width="Auto"/>
                            <ColumnDefinition Width="Auto"/>
                            <ColumnDefinition Width="*"/>
                        </Grid.ColumnDefinitions>
                        <Grid.RowDefinitions>
                            <RowDefinition Height="Auto"/>
                            <RowDefinition/>
                        </Grid.RowDefinitions>
                        <ToggleButton x:Name="Expander" Style="{StaticResource ExpandCollapseToggleStyle}" ClickMode="Press" IsChecked="{Binding Path=IsExpanded, RelativeSource={RelativeSource TemplatedParent}}"/>
                        <Border SnapsToDevicePixels="true" x:Name="Bd" Grid.Column="1" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Padding="{TemplateBinding Padding}">
                            <ContentPresenter SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" x:Name="PART_Header" ContentSource="Header"/>
                        </Border>
                        <ItemsPresenter x:Name="ItemsHost" Grid.Column="1" Grid.ColumnSpan="2" Grid.Row="1"/>
                    </Grid>
                    <ControlTemplate.Triggers>
                        <Trigger Property="IsExpanded" Value="false">
                            <Setter Property="Visibility" TargetName="ItemsHost" Value="Collapsed"/>
                        </Trigger>
                        <Trigger Property="HasItems" Value="false">
                            <Setter Property="Visibility" TargetName="Expander" Value="Hidden"/>
                        </Trigger>
                        <Trigger Property="IsSelected" Value="true">
                            <Setter Property="Background" TargetName="Bd" Value="{DynamicResource {x:Static SystemColors.HighlightBrushKey}}"/>
                            <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.HighlightTextBrushKey}}"/>
                        </Trigger>
                        <MultiTrigger>
                            <MultiTrigger.Conditions>
                                <Condition Property="IsSelected" Value="true"/>
                                <Condition Property="IsSelectionActive" Value="false"/>
                            </MultiTrigger.Conditions>
                            <Setter Property="Background" TargetName="Bd" Value="{DynamicResource {x:Static SystemColors.ControlBrushKey}}"/>
                            <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
                        </MultiTrigger>
                        <Trigger Property="IsEnabled" Value="false">
                            <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

And all this time all I really wanted to do was to get rid of the border around the ToggleButton (as commented out) !!!!!
Thx for the help. Don't think I'd have got there without it,
Best,
Viv