So, 5 weeks after approval in Texas with regular calls to the NVC to see if our file has been transferred, I rang the USCIS customer service number to be told that they wouldn't discuss the case with the beneficiary, but only the applicant. My fiancee called them and was advised that our file had gone directly to the embassy. I have now paid the courier fee and am awaiting confirmation of an interview date. All this without the magic number we were led to believe was a prerequisite for any activity at the embassy.
I'm delighted (obviously) but frustrated to know that the file has been sitting in London for 3 or 4 weeks without us knowing.