I thought I'd write a bit of an update on how I'm getting on with the D780.
I've now shot about 3,600 images with the D780 and you're right Dave, I'm not going to miss the D750. I feel sad saying that as it was a reliable workhorse of a camera that stood me in good stead. The D780, however, is a definite upgrade with better auto-focus, far superior live view, fantastic high-ISO performance, faster frame rate coupled with bigger/faster buffer and, to my mind, better control layout. I haven't found any problem learning the new control positions and can already hit the right button first time without taking my eye from the view finder, something that never really happened with the D750.
My keeper rate for wildlife pictures in particular has improved exponentially and my confidence in the AF system is almost total. I still need to improve my framing and composition of birds in flight but no longer have any lingering doubt that the image will be sharp enough.
I always got a good number of shots from one battery charge on the D750 - typically between 2,000 and 2,500 - but the D780 is even better. Last Thursday I was at a charity track day and shot 2,030 images and the battery is still showing 43% left. Now that may drop off quickly but I would say something close to 3,500 is likely. The battery is an EN-EL15c, by the way. I also managed to almost fill two memory cards (32GB each with slot 2 set to overflow) so I've forked out for a 64GB one for slot 1 - that's £100 I wasn't planning to spend
I'll get another one IF I find I need it.
My conclusion so far is that, as a highbred camera, the D780 is in a class of it's own