CheckBoxList problem

Hi
Whilst trying to fathom this problem I have narrowed it down to the following.
I have 2 checkboxlists in an updateable panel:
<asp:CheckBoxList ID="cblWeek" runat="server" AutoPostBack="true" OnSelectedIndexChanged="PressSelection" ></asp:CheckBoxList>



<asp:CheckBoxList ID="CheckBoxList1" runat="server" AutoPostBack="true" OnSelectedIndexChanged="PressSelection" >
<asp:ListItem></asp:ListItem>
<asp:ListItem></asp:ListItem>
<asp:ListItem></asp:ListItem>
<asp:ListItem>< ...