Apologies for the duplicates but after a few hours of guessing because there isn't enough material on the web for this kinda stuff, the solution is:
</DataGridTemplateColumn.CellTemplate>
<DataGridTemplateColumn.CellEditingTemplate>
<DataTemplate>
<ComboBox ItemsSource="{Binding Source={StaticResource DeploymentTypeEnum}}"
SelectedItem="{Binding DeploymentType}"
**IsSynchronizedWithCurrentItem="false**">
</ComboBox>
</DataTemplate>
</DataGridTemplateColumn.CellEditingTemplate>
IsSynchronizedWithCurrentItem
- does what it says on the tin. However, when you select an item, the current one will disappear but at least it won't update all rows.