Yep, it's setup just as I always used to via the render layer mask IDs.

The tag has the ID number selected...
And I have setup the render as it should be right?

Please can someone help, this is driving me nuts.

All worked in the past, then updated. Now when I select the chosen layer ID on the Object tag, then go to select it in the pass settings, nothing renders.

I just want white mattes. Now there is an 'instance ID' since updating? Not sure if this is ...