You all supported me, I owe you all a report.
The good news is that thanks to Espen Beck who is the product manager for digital backs at Phase one, and Olivier Percheron who is the technician at Prophot, I have a replacement new P45+ back which is a definite improvement on the first one.
The bad news is that I have adjusted my expectations of what one can expect of a P45+. It appears some degree of striation at high ISO is normal for this product.
Good news detail: When I got to Prophot, Olivier had already read the LL thread and had got some phone calls. He opened a new back, and did some comparison images of me wearing my dark grey wool sweater. The first images he took of me at ISO 800 immediately showed up the centerfold issue. I was offered the just-opened back as a warranty replacement and accepted this offer.
Bad news detail: We did some comparison tests with the old and new back at 800,400 and 200 ISO. The new back too shows some -a few- striations in the shadows, although fewer than the old one, and these bands are really apparent at ISO 800 and 400, and you can still see them at ISO 200 if you know what you are looking for. This is bad news because it appears to be a general characteristic of the product.
Olivier and I agree, that after comparing the old and new backs we can say that the old one was out of spec (centerfold), but that with respect to the new one I have to be prepared to adjust my expectations, and accept that shots above ISO 200, or underexposed shots at ISO 200 cannot be expected to be really completely clean. Olivier states the real native ISO of this product is ISO 50, and we would agree that it should be shot at or below ISO 200, with ISO 400 and 800 risking a need for some retouching in the shadows, although if lucky the pictures would be ok.
It also appears therefore, that the common advice of using the P30+ for low light shooting is probably justified because it should shoot ISO 400 without striations, if it has the same performance as the P45+ with one stop more due to microlenses.
In the end Phase did indeed deliver on its warranty - I do regret that my hopes of smooth albeit noisy Hi-ISO have been dashed for the time being, although one can always hope for future improvement. I will temper my expectations, and await your reports on competing backs at high ISO with great interest.
This has certainly been a learning experience, and I am glad it is resolved. I would like to thank Espen Beck at Phase and Olivier Percheron at Prophot for their friendly help, my dealer Fabrice Michaux at Le Moyen Format for his solidarity, and everyone who contributed to this thread for their support and encouragement which certainly helped to get this problem solved.