Just to update you on my findings: I received the replacement yesterday and was able to compare both cameras side by side. As I suspected might be the case, the image noise on both cameras was identical, and the amount of noise I'm seeing at 0dB would seem to be standard for the Z7. Perhaps I'm expecting too much, but I don't feel that I am for a £4000+ camera. For me, what I see really is as if Sony had simply renamed 0dB as -6dB. In all other respects I am very happy with the camera so far, and it seems that I'm just going to have to live with it. It does mean that I will be shooting at -6dB wherever possible, which is annoying as it negates the improved low light performance the camera is supposed to offer.

I concurr. I was just demoing some o my recent projects shot with the Z7 and definitely noticed that the finished product, compared to similar Z1 projects, with the same treatment shows that the Z7 footage is just as noisy . To me the grain pattern is tighter, but still very noisy.