OK I worked out what was happening.
data() was returning a checkbox AND a value. To stop it, I had to modify it:

Qt Code: 
  1. QVariant PurchaseOrdersRequests::data ( const QModelIndex & index, int role ) const
  2. {
  3.     QVariant value = QStandardItemModel::data(index, role);
  4.     if (index.column() == 6)
  5.     {
  6.         if (role == Qt::CheckStateRole)
  7.             return (QStandardItemModel::data(index).toInt() != 0) ? Qt::Checked : Qt::Unchecked;
  8.         else if (role == Qt::DisplayRole)
  9.             return QVariant(); // Don't show value !!
  10.     }
  11.     return value;
  12. }
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