Thanks for all the comments and pictures. Less than 400-500mm focal length doesn't meet my needs and 600mm is even better, all of which puts me into the market for a Sigma 'A' mount 150-500mm or 150-600mm DG OS as this is the only long lens with image stabilisation. If I drop the need for IS then Sony/Minolta or Tamron is also possible including the excellent Sony 70-400mm G lens.
However, a comment elsewhere makes me think about buying an A6000 instead of a longer lens. Just using the APS option on the A7 offers no image benefit (just crops automatically to about 10Mpx) though it makes framing a bit easier but a good APS camera with the 300mm lens which I already have is pretty much as good as a 450mm lens on full frame. It could be a reasonably good option if the ergonomics and focusing are OK. Cost wise it will be much the same or less and will be lighter to carry.
I just don't like the idea of multiple bodies, lenses don't drop in value as fast as bodies do.