Vista - Dynamically setting RadioButtonList item as selected

This seems so trivial but I've spent the best part of a day trying to
solve. Please help!

I have a radiobuttonlist control on a web page with a DataSourceID set
to a stored procedure

<asp:RadioButtonList ID="RadioButtonList1" runat="server"
AppendDataBoundItems="false"
DataSourceID="SQLDataSource1" DataTextField="Text"
EnableViewState="true">
</asp:RadioButtonList>

It lists possible answers to a question. Think multiple choice exam;
select radio button to make your selection.

On the page I have Previous and Next buttons to scroll through
questions. These record the selected item in the database and display
the previous or next question as appropriate. No problem with any of
this.

I want the page to display the item they chose if the user views a
question already answered. I successfully retrieve the data item from
the database but I get an out of range error

"Index was out of range. Must be non-negative and less than the size
of the collection"

when i try

RadioButtonList1.Items(nUserChoice).Selected = True

If I comment out this line, all choices are displayed and I have tried
entering constant values of 0 and 1 in the above line of code but the
same error message appears.

I am attempting this action in the onclick event of the button after
rebinding the radiobuttonlist control, i.e.

RadioButtonList1.Items.Clear()
SQLDataSource1.SelectParameters("TIID").DefaultValue = TIID
SQLDataSource1.SelectParameters("QID").DefaultValue = QID
SQLDataSource1.DataBind()
RadioButtonList1.Items(nUserChoice).Selected = True

As I said, the data displays and behaves correctly otherwise. I must
be doing something stupid but I just can't see it.