RESOLVED: set a node title programmatically in node_form validate

I'm going slowly and quietly mad here. Any advice would be greatly appreciated.

This concerns form processing on the node_form for a particular content type.

I have a rather peculiar use case to satisfy.

I have a content type in which I have a node reference field. The logic I need to enact is:

I am trying to do this in my validate function. As follows:
MYMOD_contenttype_node_form_validate(&$form_state, &$form, $form_id) (Call by reference on the first two vars).

And I have prepended my validate function to the form's #validate array, so presumably it fires first.

So far, I have had to set $form['title']['#required'] = FALSE; in my form_alter to do this.

I have tried testing with:
form_set_value($form['title'], "This is a test value", $form_state);
and
$form_state['values']['title'] = "This is a test value";
and with and without
$form_state['rebuild'] = TRUE;

Nothing seems to work. If I don't do a form_set_error to stop the processing I end up with a node that has no title.

There HAS to be some way to make this work.

Any advice would be much appreciated.

Thanks for reading.