Hi there and thanks for the reply.
I bought the car from an independent dealer, not a Honda dealer, so the warranty is not a Honda warranty. I guessed that the same thing might be true of the independent dealer's warranty, though, so I checked with the dealer before I had other mechanics look at the car. With the permission of the dealer, my local mechanic re-alligned the steering and checked the rack and steering motor. He advised replacement of both. With the knowledge of the dealer, I have since had a power steering specialist check the car, and he agrees that I should not be driving it as it is. I haven't had a chance to call the dealer with this news, but I will do so tomorrow morning. The power steering specialist has also advised replacement of the steering assembly. I am hoping the warranty will cover the costs involved and that the dealer will authorise a mechanic local to me to carry out the work, as I do not want to drive for over an hour on the motorway in a car that a power steering specialist has described as unsafe.
Thanks again for your reply. I have also found a post on another forum where a Jazz owner describes the same problem and having the steering assembly replaced by her Honda dealer who said it was something they had done several times. I will print that post as well as these to show my dealer if necessary.