Hey Graeme,
Item template part is empty in your case. Can you please look into below snap.
<ItemTemplate>
<table style="width: 275px" cellspacing="0" cellpadding="0">
<tr>
<td style="width: 175px;">
<%# DataBinder.Eval(Container, "Id")%>
</td>
<td style="width: 60px;">
<%# DataBinder.Eval(Container, "Name")%>
</td>
</tr>
</table>
</ItemTemplate>
also you can check this link for more info
Demos of Telerik ASP.NET ComboBox control[
^]