Sorry, my original post wasn't clear (even to me, as it seems I forgot how to speak English during some of it).
I have no problem getting hold of the string representation of the field name from the GridView, the problem I'm having is in constructing the ISortExpression in my code. I need to do something like this:
protected void SortGrid(string sortFieldName)
{
....
SortExpression sorter = new SortExpression(new SortClause(fields[sortFieldName], SortOperator.Ascending))
....
return dao.GetAsDataReader(...);
}