Thanks for your comments.
In reply to Judd. Until the rack was removed it was not known what the exact problem was and sensors were one of the possible causes. Consequently rack and sensors were tested/inspected by the engineering company and the answer lies in the photo's. They supplied a different refurbished rack and returned the old one as I wanted to see whether it was a manufacturing defective part, with a view to contacting Subaru, which it is in the opinion of various mechanics and the engineering company..
The vehicle has been serviced according to the manual by Subaru dealerships, until this year when it was serviced and MoT'd by a local garage (who service a very large haulage company recently featured on TV). The nearest Subaru dealership is in Durham and due to Covid restrictions causing a huge backlog they could not carry out a service/MoT for many weeks.
As regards salt. Neither the previous owner, who I know, nor me, live anywhere near the sea, have not used the car off road and have avoided using the car in winter on salted/gritted roads unless absolutely necessary.
I'm not a mechanic or technical expert any any means, but I thought the rusted pinion fitted inside the steering column. If it doesn't then why is that the only rusted part of the rack and if it does - same question.