Thought I would post a little update on the developments in our case this week in case it is helpful to anyone or if someone has had a similar experience...?
We are now approximately Day 77 (we received our confirmation email from Sheffield on 4th April 2017 and are now way past the average 60BD processing window).
Following advice given on this forum, I contacted my local MP's office and they have been trying to assist us in getting some answers from Sheffield. They made contact with 2 different people on the MP hotline:-
The first said that UKVI were looking into whether they needed my actual UK passport, rather than just the scanned copy they requested. (This seems unlikely to me as we have been told previously that they have all the necessary documentation for our case.) We have not heard anything further regarding this and no actual request for my passport has arisen.
The second said that "there is a lot more paperwork to go through for our case, and it is more complicated" and would not commit to a timeline. Again this seems strange, as to me our case is really straightforward. We are a legally married couple (one Brit, one American), one biological child born February 2017 who is a dual citizen, no criminal records, job offers awaiting us in UK, and we meet the financial requirements requested.
I also called the helpline again last week, and they have "escalated" our case which should mean an answer within 15BD. Apparently they could have done this after our call at 60BD but we obviously spoke to the wrong person that time because this has only just happened now due to our persistence!
In the meantime we try to stay positive, and calm and hope that there is a light at the end of what feels like a very long, very dark tunnel!