After a lot of running into dead-ends thinking I could resolve this by changing the ItemsPresenter or ItemsPanel, I ended up using a DockPanel within my ItemTemplate, instead of the grid:-
<Border BorderThickness="0,0,0,1"
BorderBrush="LightGray"
Padding="0,2,0,2">
<DockPanel LastChildFill="true">
<NumericUpDown DockPanel.Dock="Right"
Width="75"
Text="{Binding ...}" />
<TextBlock Text="{Binding ...}" />
</DockPanel>
</Border>
I'm not entirely sure why the DockPanel respects the size of the parent ItemsControl, while the Grid was happy to horizontally "overrun" off the edge. This is an aspect of WPF that I still struggle with - which controls' sizes are affected by their parent, and which are affected by their children's sizes!