Forms Data Controls :: A Field Or Property With The Name 'FieldName' Was Not Found On The Selected Data Source?

I'm attempting to load a GridView dynamically on a button is pressed.The gridview has AutoGenerateColumns="True" (set in skin definition) and has a declarative column definitions as follows:

<asp:GridView runat="server"
SkinID="GridViewDetail" ID="gvReport"
DataKeyNames="dateofcompletion">
<Columns>
[code]...

This all works fine the first time, but as soon as I select a different ddlCampaign item, it throws the"A field or property with the name 'FieldName' was not found on the selected data source" ERRORI kinda understand that it has retained the column mappings from the previous datasource assignment/binding and because the new datasource has different column headings it is throwing the error. What I can't seem to achieve is a clear or reset of defined gridview columns before adding the new columns. I cannot clear all columns because part of the GV defn is declarative.e.g.

gvReport.Columns.Clear();
gvReport.AutoGenerateColumns = true;

I have also tried clearing the previously added columns before new assignment/binding but it gives the same error.e.g.

for (int i = 2; i < gvReport.Columns.Count; i++)
{
gvReport.Columns.RemoveAt(i);
}