Further investigation reveals this code for the hidden form element for the record ID:
<input type="hidden" name="WADAUpdateRecordID" id="WADAUpdateRecordID" value="<?php echo((isset($_GET["invalid"])?ValidatedField("testupdate","val"):$_GET["SANITIZETHEID"])); ?>" />
Not sure where: ValidatedField("testupdate","val") comes from - it doesn't seem valid.