Thank you for your suggestions.
I am (or was) using the latest firmware and Phocus software for the back. This, I checked and reformatted, twice. I even reset it as per Hasselblad's instructions.
I tested the back at the sellers house with his camera and saw no problem as I was stupid enough not to shoot a blank wall to look at the results. It was only when I had chance to really look at the files at home on my big screen did I see the hairline row error. When I tested the back tethered in live view on a blank wall it was very obvious.
I tried setting the back at all of the iso settings and the problem was still evident in live view. Various different Hasselblad agents or Hasselblad themselves have got back to say.
"...from the jpeg & description of error it sounds & looks like a column error.
If its ONLY a row error, then this can be easily corrected via a re-calibration.
However worse case scenario if it cant be corrected, would mean possibly a new sensor ...which we dont have for CFV16's any longer.
I would need to see a FFF file minimum to make a better judgment.
Hasselblad Technical Specialist"
I have now returned the back at great expense to myself to the owner and wasted a lot of money on travel and postage to buy and return it.
Although the seller is saying he tested the back upon it's return and there is no problem he refuses to show me a raw file to prove what he states.
I would warn anyone else if they are approached by an individual who is selling a CFV 16 II with an external serial number of 3DSR12121 do so with your eyes open and the knowledge the back may be compromised.