Thank you so much for taking the time to show that.
When I looked a couple of days ago I was seeing return fares to NY in Nov for £2400 in J on ba.com. £1200 each way for eg the 9.30pm flight. I didn't think to see if I could replicate it on ITA Matrix. So the £1200 plus approx £800 you saw was what I was hoping to find. Today the Nov flights I'm looking at are £2,500 each way to and from NY! Something must be going on then... and maybe that is why it wasn't pricing it up properly, maybe the I class or whatever I'd selected was literally getting sold out. I certainly can't see it now!
Next time I'll start on ITA Matrix that might be the better first step. Thank you for this, much appreciated.