Had to delay the trip few weeks and got back yesterday. I thought you might want to know how it went.
In a word "Perfect"!
Although the plane pushed back late it was a quick flight to LAX at 10h15m and we arrived on time, and more importantly just in front of the Air France A380 but behind the LH A340. From doors open at around 13.15 we cleared immigration and were through to the domestic terminal via the transit route with it's own TSA security within an hour. That's much, much better than I expected, And the bags made it to our final destination as well although the laid back "just leave them there" instruction from the baggage handler left us wondering whether they would.
And for some reason all our party were given TSAPRE on our return domestic sectors, an on the BA return from IAD to the UK which was the fastest security check I have experienced for what I guess is 30 or so years. One has to wonder why the airports and airlines on this side of the pond can't persuade the DfT to allow a similar pre clearance process here.
Thanks again to all who contributed