Well, after much contemplation and internal debate I decided that the 70-200 f/4 IS, the 400 f/5.6 and a 1.4x extender is my preferred combination. I really debated between that and f/2.8 and a 2x extender. In the end the lighter weight of the f/4 and better quality of the prime lens won out over the extra f stop of aperture. I realised that if I lifted the camera and existing lens and also carried my 20mm T2 Nagler it would weigh about the same as the camera and f/2.8. It was too much for comfort and too much to hang around my neck. The combination I've chosen is heavier overall but you have less around your neck at any time.
Unfortunately (or is that fortunately) I've spent quite a bit on the troopie recently and we're about to go away on hols and spend more, so I couldn't buy both lenses straight off. The 70-200 and the 1.4x are on the way and I'll have to get the 400 when the bank recovers.
Thanks to everyone for their comments - it really did help.
