Thank you for the replies. I have decided to go for the existing 600/II once the sale (I hope) goes through. One more question:
Which would you take on a safari in a private game reserve in South Africa?
a) 1Dx2 with 200-400/1.4x + 5D4 with 70-200/2.8 II or
b) 1Dx2 with 600/4 II + 5D4 with 100-400 II
Thank you again for the advice.
Much of the time on safari you are sitting in a Jeep and you don't get out with a tripod and it isn't simple to use a bean bag - you have to hand hold and cope with a range of distances and angles. The 100-400 was the ultimate safari lens, with a very useful zoom range and easy to pack and swing around. The 100-400mm II takes its place, and a Sigma 150-600mm C (or Tamron) is a good choice as well. You need to be able to cope with a range of distances. I personally would take the 400 DO II plus extenders, which is easy to maneuver, with my wife having the 100-400mm II.
5D IV, 5DS R, 7D II, EOS-M, Powershot G3 X, 400mm DO II, 300/2.8 II, 1.4xTC III, 2xTC III, EF 1.8 STM, EF 70-200/4 IS, EF 24-105, 15-85, 100-400 II, Sigma 10-20, Sigma 150-600mm C, EOS-M5 15-45, f/2 22, 11-22, Samyang 8mm f/2.8 fisheye