blocking up external doorway: cavity tray height?

[rsk1]

Another cavity tray question

I am blocking up an external doorway, brick with insulated cavity. Floor level and ground level are the same.
Since the cavity doesn't extend 225 below floor level, I guess I need a cavity tray and weep vents. What does BR have to say about cavity trays? can i use visqueen at 150 height in the outer leaf (forming the dpc), rising to the next course up in the inner leaf?
Can insulation sit in the cavity tray, and does the space under the tray remain empty and uninsulated?

And how should the dpc's interact with the walls on either side : it's a victorian house so no idea where the dpc is in the old walls

 

[noseall]

If you are worried about driven rain penetration or otherwise, turn some plastic up and over the bottom course of bricks - internally and under the external (DPC) course.
If the lintel remains above and you have a sub-DPC trough (unlikely with a block-up) then I would not bother with a low level CT.