The Post Office records are currently being transcribed by Ancestry and should be available sometime later this year.
The Post Office Archive is situated in Pheonix Place, just behind the Mount Pleasant Mail Centre, you can google for address & phone number.
Do call them, they may be able to provide the records you want for a fee, not too sure if they do this service but you can but ask.
They are open at least 2 Saturdays a month. But I have also heard that they could be moving out of London sometime (though this has been talked of before in the past).
