ok i think i have this solved.
in the MRT behaviour i set the for the master key to the $_POST['WADAUpdateRecordID"]
when in fact it should of been using either $_GET['id_sec']
The update of the record was working
because the $_POST['WADAUpdateRecordID"] variable did indeed contain the correct value.
But when the page is first loaded, this data has not yet been set, as the update button needed to be pressed, only then would it have been avaliable to the page.
i feel a bit stupid having wasted so much time on this, but pleased i figured it out on my own :)